Simultaneous multi-state read or verify in non-volatile storage

ABSTRACT

Methods and devices for simultaneously verifying or reading multiple states in non-volatile storage are disclosed. Methods and devices for efficiently reducing or eliminating cross-coupling effects in non-volatile storage are disclosed. Methods and devices for efficiently performing reads at a number of voltages to search for the threshold voltage of a memory cell are disclosed. Memory cells on different NAND strings that are read at the same time may be tested for different threshold voltage levels. Memory cells may be tested for different threshold voltages by applying different gate-to-source voltages to memory cells being tested for different threshold voltages. Memory cells may be tested for different threshold voltages by applying different drain to source voltages to the memory cells. Different amounts of compensation for cross-coupling affects may be applied to memory cells on different NAND strings that are read or programmed at the same time.

CLAIM OF PRIORITY

This application is a divisional application of U.S. patent applicationSer. No. 12/732,121, entitled “SIMULTANEOUS MULTI-STATE READ OR VERIFYIN NON-VOLATILE STORAGE,” filed Mar. 25, 2010, which is incorporatedherein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This disclosure relates to non-volatile memory.

2. Description of the Related Art

Semiconductor memory has become increasingly popular for use in variouselectronic devices. For example, non-volatile semiconductor memory isused in cellular telephones, digital cameras, personal digitalassistants, mobile computing devices, non-mobile computing devices andother devices. Electrically Erasable Programmable Read Only Memory(EEPROM) and flash memory are among the most popular non-volatilesemiconductor memories. With flash memory, also a type of EEPROM, thecontents of the whole memory array, or of a portion of the memory, canbe erased in one step, in contrast to the traditional, full-featuredEEPROM.

Both traditional EEPROM and flash memory utilize a floating gate that ispositioned above and insulated from a channel region in a semiconductorsubstrate. The floating gate is positioned between the source and drainregions. A control gate is provided over and insulated from the floatinggate. The threshold voltage (V_(TH)) of the transistor thus formed iscontrolled by the amount of charge that is retained on the floatinggate. That is, the minimum amount of voltage that must be applied to thecontrol gate before the transistor is turned on to permit conductionbetween its source and drain is controlled by the level of charge on thefloating gate.

Some EEPROM and flash memory devices have a floating gate that is usedto store two ranges of charges and, therefore, the memory element can beprogrammed/erased between two states, e.g., an erased state and aprogrammed state. Such a flash memory device is sometimes referred to asa binary flash memory device because each memory element can store onebit of data.

A multi-state (also called multi-level) flash memory device isimplemented by identifying multiple distinct allowed/valid programmedthreshold voltage ranges. Each distinct threshold voltage rangecorresponds to a predetermined value for the set of data bits encoded inthe memory device. For example, each memory element can store two bitsof data when the element can be placed in one of four discrete chargebands corresponding to four distinct threshold voltage ranges.

Typically, a program voltage V_(PGM) applied to the control gate duringa program operation is applied as a series of pulses that increase inmagnitude over time. In one possible approach, the magnitude of thepulses is increased with each successive pulse by a predetermined stepsize, e.g., 0.2-0.4 V. V_(PGM) can be applied to the control gates offlash memory elements. In the periods between the program pulses, verifyoperations are carried out. That is, the programming level of eachelement of a group of elements being programmed in parallel is readbetween successive programming pulses to determine whether it is equalto or greater than a verify level to which the element is beingprogrammed. For arrays of multi-state flash memory elements, averification step may be performed for each state of an element todetermine whether the element has reached its data-associated verifylevel. For example, a multi-state memory element capable of storing datain four states may need to perform verify operations for three comparepoints.

U.S. Pat. No. 7,073,103, entitled “Smart Verify For Multi-StateMemories,” incorporated herein by reference in its entirety, describes aprocess for minimizing the number of sequential verify operations foreach program/verify/lockout step of a write sequence. Initially, onlythe lowest state of the multi-state range to which selected storageelements are programmed is checked during the verify phase. Once thefirst storage state is reached by one or more of the selected elements,the next state in a sequence of multi-states is added to the verifyprocess. This next state can either be added immediately upon thefastest elements reaching the preceding state in the sequence, or aftera delay of several program pulses. The adding of states to the set beingchecked in the verify phase continues through the rest of the set ofmulti-states in sequence, until the highest state has been added.Additionally, lower states can be removed from the verify set as all ofthe selected storage elements bound for these levels verify successfullyto those target values and are locked out from further programming. Notethat this technique may require that more than one state be verifiedfollowing each programming pulse.

While methods for improving the verify technique are known, a need stillexists for further improvements which are applicable to differentprogramming schemes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a top view of a NAND string.

FIG. 1B is an equivalent circuit diagram of the NAND string of FIG. 1 a.

FIG. 2 is a circuit diagram depicting three NAND strings.

FIG. 3 depicts a cross-sectional view of a NAND string formed on asubstrate.

FIG. 4 illustrates a non-volatile storage device that may include one ormore memory die or chips.

FIG. 5 depicts an exemplary structure of memory cell array.

FIG. 6 is a block diagram of an individual sense block.

FIG. 7A depicts example threshold voltage distributions for states ofmemory cells in which there are eight states.

FIG. 7B illustrates example threshold voltage distributionscorresponding to data states for the memory cell array when each memorycell stores four bits of data.

FIG. 8A depicts an example set of threshold voltage distributions for afour-state memory device in which each storage element stores two bitsof data.

FIG. 8B depicts a series of program and verify pulses which are appliedto a selected word line during a programming operation.

FIG. 9A is a schematic diagram of a circuit that is able to applydifferent gate-to-source voltages across a selected memory cell ondifferent strings of memory cells.

FIG. 9B is a schematic diagram of NAND strings that have differentvoltages applied across them during a read operation.

FIG. 9C is a schematic diagram of NAND strings that have differentvoltages applied across them during a verify (or read) operation.

FIG. 10 is a flowchart of one embodiment of a process of sensingconditions of memory cells in different NAND strings by applyingdifferent voltages across different NAND strings.

FIG. 11A is a block diagram of one embodiment of a sense module forcharging a bit line to a voltage that is lower than the source linevoltage.

FIG. 11B is a block diagram the sense module of FIG. 11A showing thedirection of current when sensing a memory cell's threshold voltage.

FIG. 12A is a block diagram of one embodiment of a sense module forcharging a bit line to a voltage that is higher than the source linevoltage.

FIG. 12B is a block diagram the sense module of FIG. 12A showing thedirection of current when sensing a memory cell's threshold voltage.

FIG. 13A is a flowchart of one embodiment of a process of determining asuitable voltage to apply across a NAND string based on the state beingverified.

FIG. 13B is a flowchart of one embodiment of a process of determining asuitable voltage to apply across a NAND string to compensate forcross-coupling during read.

FIG. 13C is a flowchart of one embodiment of a process of determining asuitable voltage to apply across a NAND string to compensate forcross-coupling during programming.

FIG. 13D is a flowchart of one embodiment of a process of determining asuitable voltage to apply across a NAND string based on a previousthreshold voltage of the memory cell.

FIG. 14A is a flowchart of one embodiment of a process for establishingdifferent voltages across NAND that causes different Vgs for selectedmemory cells on different NAND strings.

FIG. 14B is a flowchart of one embodiment of a process for establishingdifferent voltages across NAND strings that causes different Vds forselected memory cells on different NAND strings.

FIG. 15 is a flowchart describing one embodiment of a programmingprocess, which includes one or more verification steps.

FIG. 16A is a flowchart of one embodiment of a process of verifyingmemory cells during a programming operation.

FIG. 16B is a timing diagram showing one embodiment of voltages appliedto word line and bit lines during the verify process of FIG. 16A.

FIG. 17A is a flowchart of one embodiment of a process of verifyingmemory cells during a programming operation.

FIG. 17B is a timing diagram showing one embodiment of voltages appliedto word line and bit lines during the verify process of FIG. 17A.

FIG. 18A is a diagram of several NAND strings showing example voltagesapplied when verifying the C-state in the process of FIG. 17A.

FIG. 18B is a diagram of several NAND strings showing example voltagesapplied when verifying the B-state in the process of FIG. 17A.

FIG. 18C is a diagram of several NAND strings showing example voltagesapplied when verifying the A-state in the process of FIG. 17A.

FIG. 19 is a diagram of one embodiment of a NAND string that has atransistor to help control Vds when performing reverse sensing.

FIG. 20 depicts a flowchart of one embodiment of a process of applyingcross-coupling compensation during reading.

FIG. 21 depicts memory cell threshold voltage distributions.

FIG. 22 is a flowchart of one embodiment of a process of reading softbits.

FIG. 23A is a flowchart of one embodiment of a process of performing abinary search.

FIG. 23B is an example window over which the search of the process ofFIG. 23A may be performed.

FIG. 24 depicts one embodiment of a sensing circuit.

FIG. 25A, FIG. 25B, and FIG. 25C depict details of operation of bit linebias transistors of FIG. 24 for a reverse sensing embodiment.

FIG. 26A and FIG. 26B depict voltages applied to sense bias transistorsof FIG. 24 for a reverse sensing embodiment.

FIG. 27 depicts a timing diagram for signals associated with the sensingcircuit of FIG. 24.

FIG. 28A depicts bit line bias transistors for a forward sensingembodiment.

FIG. 28B depicts sense bias transistors for a forward sensingembodiment.

FIG. 29 depicts a timing diagram for signals associated with a forwardsensing embodiment.

DETAILED DESCRIPTION

Methods and devices for simultaneously verifying or reading multiplestates in non-volatile storage are disclosed. Technology disclosedherein reduces the time for non-volatile storage operations such asprogram verify, and read in non-volatile.

Methods and devices for efficiently reducing or eliminatingcross-coupling effects in non-volatile storage are disclosed. Technologydisclosed herein reduces the time it takes to program or read whilereducing or eliminating cross-coupling effects.

Methods and devices for efficiently performing reads at a number ofvoltages to search for the threshold voltage of a memory cell aredisclosed. Technology disclosed herein reduces the time is takes todetermine “soft bits.”

In one embodiment, memory cells on different NAND strings that are readat the same time are tested for different threshold voltage levels. Forexample, a memory cell on one NAND string is tested to determine whetherits threshold voltage is above a first voltage level, whereas a memorycell on another NAND string is tested to determine whether its thresholdvoltage is above a second voltage level. As a specific example, onememory cell is tested to determine if its threshold voltage is above thevoltage level associated with one data state, whereas, the other memorycell is tested to determine if its threshold voltage is above thevoltage level associated with another data state. Therefore, in a memoryarray that programs memory cells to multiple bits per memory cell, morethan one data state can be tested at a time.

In one embodiment, the technique is used during a verify operationassociated with programming memory cells to multiple data states. Byverifying multiple data states at a time, the programming sequence issped up. For example, one or more programming pulses are applied to thememory cells with the goal of eventually programming some memory cellsto one state and others to a different state (in all there might be 4,5, 6, 7, 8, 9, . . . , 16, or more states). Then, a verify operation isperformed to determine whether the memory cells were programmed to theirrespective intended states as a result of the one or more programmingpulses. By verifying multiple states with the same verify operation,time may be saved. In some embodiments, the same voltage is applied tothe gates of the memory cells that are being verified for differentthreshold voltages.

In one embodiment, memory cells are tested for different thresholdvoltages by applying different gate-to-source voltages to memory cellsbeing tested for different threshold voltages. One embodiment involvesbiasing a common source line of a NAND string to a first voltage andcausing one voltage to a first set of bit lines and a different voltageto a second set of bit lines. The first set of bit lines are associatedwith NAND strings having a memory cell being programmed to a first stateand the second set of bit lines are associated with NAND strings havinga memory cell being programmed to a second state. However, the samevoltage is applied to the gates of the memory cells being programmed.The bit line voltages may be lower than the common source line suchthat, in effect, the side of the memory cell nearest the source linefunctions as the drain and the side of the memory cell nearest its bitline functions as the source. Therefore, the memory cell's current flowsfrom the source line to its bit line. This technique may therefore bereferred to as “reverse sensing” in that the current flows in theopposite direction as is commonly done to sense memory cells on a NANDstring.

In one embodiment, memory cells are tested for different thresholdvoltages by applying different drain to source voltages to the memorycells. One embodiment involves biasing a common source line of a NANDstring to a first voltage and causing one voltage to a first set of bitlines and a different voltage to a second set of bit lines. The firstset of bit lines are associated with NAND strings having a memory cellbeing programmed to a first state and the second set of bit lines areassociated with NAND strings having a memory cell being programmed to asecond state. However, the same voltage is applied to the gates of thememory cells being programmed. The different bit line voltages causedifferent drain-to-source voltages for memory cells being tested fordifferent threshold voltages. The drain voltage of at least some of thememory cells is sufficiently high to cause drain induced barrierlowering (DIBL), which alters the threshold voltage of the memory cell.For example, the goal may be to verify that the threshold voltage of afirst memory cell is at least 3.0V and the threshold voltage of a secondmemory cell is at least 3.5V. In fact, the first memory cell may have athreshold voltage of 3.0V and the second memory cell may have athreshold voltage of 3.5V without any DIBL effect. By creatingsufficient DIBL to lower the threshold voltage of the second memory cellfrom 3.5V to 3.0V, the threshold voltage of both memory cells can betested using the same gate to source voltage. As an example, under theforegoing conditions, a Vgs of 3.0V may cause both memory cells to turnon, thereby verifying memory cells having different threshold voltagesat the same time. Note this means that the same voltage is applied tothe gates of the memory cells that are being verified for differentthreshold voltages.

In one embodiment, a different amount of compensation for cross-couplingaffects is applied to memory cells on different NAND strings that areread at the same time. The compensation may be based on the thresholdvoltage of one or more neighbor memory cells. For example, a memory cellon one NAND string receives a first amount of cross-couplingcompensation, whereas a memory cell on another NAND string that is beingread at the same time receives a second amount of cross-couplingcompensation. As a specific example, the different NAND strings have adifferent amount of voltage applied across them to achieve a differentamount of cross-coupling compensation. In one embodiment, differentmemory cells have a different Vgs during read to achieve a differentamount of cross-coupling compensation. In one embodiment, differentmemory cells have a different Vds during read to achieve a differentamount of cross-coupling compensation.

In one embodiment, a different amount of compensation for cross-couplingaffects is applied to memory cells on different NAND strings during theverify stage of programming. The compensation may be based on theintended state to which a neighbor memory cell is to be programmed at alater time. For example, a memory cell on one NAND string receives afirst amount of cross-coupling compensation, whereas a memory cell onanother NAND string that is being verified at the same time receives asecond amount of cross-coupling compensation. As a specific example, thedifferent NAND strings have a different amount of voltage applied acrossthem to achieve a different amount of cross-coupling compensation. Inone embodiment, different memory cells have a different Vgs duringverify to achieve a different amount of cross-coupling compensation. Inone embodiment, different memory cells have a different Vds duringverify to achieve a different amount of cross-coupling compensation.

In one embodiment, a search for the threshold voltage is performedefficiently by performing reads at different threshold voltages at thesame time. For example, a read is performed at the center of a windowthat covers a range of threshold voltages of interest. This read may beperformed on many memory cells on a selected word line. Then, based onthe outcome of the read, another read is performed at multiple thresholdvoltages. For example, if a given memory cell's threshold voltage isdetermined to be higher than the center of the window, then that cell isread at the midpoint between the center and the top of the window. Onthe other hand, if a given memory cell's threshold voltage is determinedto be lower than the center of the window, then that cell is read at themidpoint between the bottom of the window and the center. In effect, abinary search is performed for the threshold voltage. In one embodiment,different NAND strings have a different amount of voltage applied acrossthem to read for different threshold voltages at the same time to speedup the search. In one embodiment, different memory cells have adifferent Vgs to read for different threshold voltages at the same timeto speed up the search. In one embodiment, different memory cells have adifferent Vds to read for different threshold voltages at the same timeto speed up the search.

In one embodiment, soft bits are efficiently determined by performingreads at different threshold voltages at the same time. Determining softbits allows the threshold voltage of the memory cells to be determinedto a finer level of granularity. As one example, if there are four datastates, initially three separate reads may be performed with each readbeing at a point between two adjacent data states. These initial readsdo not determine the soft bits, but instead may make an initialassignment of the threshold voltage of each memory cell to one datastate. Then, two reads may be performed together at offsets from each ofthe first reads. For example, one read is at V−Δ and the other at V+Δduring the same read operation. The data from these reads may be used todetermine “soft bits,” which may be provided to an ECC correctionalgorithm. In one embodiment, different NAND strings have a differentamount of voltage applied across them to read for V−Δ and V+Δ at thesame time. In one embodiment, different memory cells have a differentVgs to read for V−Δ and V+Δ at the same time. In one embodiment,different memory cells have a different Vds to read for read for V−Δ andV+Δ at the same time.

Example Memory System and Operation

One example of a memory system suitable for implementing embodimentsuses a NAND flash memory architecture, which includes connectingmultiple transistors in series between two select gates. The transistorsconnected in series and the select gates are referred to as a NANDstring. FIG. 1 a is a top view showing one NAND string. FIG. 1 b is anequivalent circuit thereof. The NAND string includes four transistors,100, 102, 104 and 106, in series and sandwiched between a first selectgate 120 and a second select gate 122. Select gate 120 gates the NANDstring connection to bit line 126. Select gate 122 gates the NAND stringconnection to source line 128. Select gate 120 is controlled by applyingthe appropriate voltages to control gate 120CG. Select gate 122 iscontrolled by applying the appropriate voltages to control gate 122CG.Each of the transistors 100, 102, 104 and 106 has a control gate and afloating gate. Transistor 100 has control gate 100CG and floating gate100FG. Transistor 102 includes control gate 102CG and floating gate102FG. Transistor 104 includes control gate 104CG and floating gate104FG. Transistor 106 includes a control gate 106CG and floating gate106FG. Control gate 100CG is connected to (or is) word line WL3, (whereWL denotes “word line”), control gate 102CG is connected to WL2, controlgate 104CG is connected to WL1, and control gate 106CG is connected toWL0. In one embodiment, transistors 100, 102, 104 and 106 are eachstorage elements, also referred to as memory cells. In otherembodiments, the storage elements may include multiple transistors ormay be different than depicted. Select gate 120 is connected to selectline SGD. Select gate 122 is connected to select line SGS.

FIG. 2 is a circuit diagram depicting three NAND strings. A typicalarchitecture for a flash memory system using a NAND structure willinclude many NAND strings. For example, three NAND strings 320, 340 and360 are shown in a memory array having many more NAND strings. Each ofthe NAND strings includes two select gates and four storage elements.While four storage elements are illustrated for simplicity, NAND stringscan have thirty-two or sixty-four storage elements, for instance.

For example, NAND string 320 includes select gates 322 and 327, andstorage elements 323-326, NAND string 340 includes select gates 342 and347, and storage elements 343-346, NAND string 360 includes select gates362 and 367, and storage elements 363-366. Each NAND string is connectedto the source line by its select gates (e.g., select gates 327, 347 or367). A selection line SGS is used to control the source side selectgates. The various NAND strings 320, 340 and 360 are connected torespective bit lines 321, 341 and 361, by select transistors in theselect gates 322, 342, 362, and so forth. These select transistors arecontrolled by a drain select line SGD. In other embodiments, the selectlines do not necessarily need to be in common among the NAND strings;that is, different select lines can be provided for different NANDstrings. WL3 is connected to the control gates for storage elements 323,343 and 363. WL2 is connected to the control gates for storage elements324, 344 and 364. WL1 is connected to the control gates for storageelements 325, 345 and 365. WL0 is connected to the control gates forstorage elements 326, 346 and 366. As can be seen, each bit line and therespective NAND string comprise the columns of the array or set ofstorage elements. The word lines (WL3, WL2, WL1 and WL0) comprise therows of the array or set. Each word line connects the control gates ofeach storage element in the row. Or, the control gates may be providedby the word lines themselves. For example, WL2 provides the controlgates for storage elements 324, 344 and 364. In practice, there can bethousands of storage elements on a word line.

In some embodiments, the voltage applied to the bit line is greater thanthe voltage applied to source when reading a memory cell on the NANDstring. In some embodiments, the voltage applied to the bit line is lessthan the voltage applied to source when reading a memory cell on theNAND string. For consistency of discussion, the source line will alwaysbe referred to as a source line regardless of whether the voltageapplied to it is less than or greater than the voltage applied to thebit line.

Each storage element can store data. For example, when storing one bitof digital data, the range of possible threshold voltages (V_(TH)) ofthe storage element is divided into two ranges which are assignedlogical data “1” and “0.” In one example of a NAND type flash memory,the V_(TH) is negative after the storage element is erased, and definedas logic “1.” The V_(TH) after a program operation is positive anddefined as logic “0.” When the V_(TH) is negative and a read isattempted, the storage element will turn on to indicate logic “1” isbeing stored. When the V_(TH) is positive and a read operation isattempted, the storage element will not turn on, which indicates thatlogic “0” is stored. A storage element can also store multiple levels ofinformation, for example, multiple bits of digital data. In this case,the range of V_(TH) value is divided into the number of levels of data.For example, if four levels of information are stored, there will befour V_(TH) ranges assigned to the data values “11”, “10”, “01”, and“00.” In one example of a NAND type memory, the V_(TH) after an eraseoperation is negative and defined as “11”. Positive V_(TH) values areused for the states of “10”, “01”, and “00.” The specific relationshipbetween the data programmed into the storage element and the thresholdvoltage ranges of the storage element depends upon the data encodingscheme adopted for the storage elements.

When programming a flash storage element, a program voltage is appliedto the control gate of the storage element, and the bit line associatedwith the storage element is grounded. Electrons from the channel areinjected into the floating gate. When electrons accumulate in thefloating gate, the floating gate becomes negatively charged and theV_(TH) of the storage element is raised. To apply the program voltage tothe control gate of the storage element being programmed, that programvoltage is applied on the appropriate word line. As discussed above, onestorage element in each of the NAND strings share the same word line.For example, when programming storage element 324 of FIG. 2, the programvoltage will also be applied to the control gates of storage elements344 and 364.

FIG. 3 depicts a cross-sectional view of a NAND string formed on asubstrate. The view is simplified and not to scale. The NAND string 400includes a source-side select gate 406, a drain-side select gate 424,and eight storage elements 408, 410, 412, 414, 416, 418, 420 and 422,formed on a substrate 440. A number of source/drain regions, one exampleof which is source/drain region 430, are provided on either side of eachstorage element and the select gates 406 and 424.

In one approach, the substrate 440 employs a triple-well technologywhich includes a p-well region 432 within an n-well region 434, which inturn is within a p-type substrate region 436. The NAND string and itsnon-volatile storage elements can be formed, at least in part, on thep-well region. A source supply line 404 with a potential of V_(SOURCE)is provided in addition to a bit line 426 with a potential of V_(BL).Voltages, such as body bias voltages, can also be applied to the p-wellregion 432 via a terminal 402 and/or to the n-well region 434 via aterminal 403. During a read or verify operation, a control gate voltageV_(CGRV) is provided on a selected word line, in this example, WL4,which is associated with storage element 416. Further, recall that thecontrol gate of a storage element may be provided as a portion of theword line. For example, WL0, WL1, WL2, WL3, WL4, WL5, WL6 and WL7 canextend via the control gates of storage elements 408, 410, 412, 414,416, 418, 420 and 422, respectively. A pass voltage, V_(READ) is appliedto the remaining word lines associated with NAND string 400. V_(SGS) andV_(SGD) are applied to the select gates 406 and 424, respectively.

Note that the direction current flow along the NAND string may depend onthe relative magnitudes of V_(SOURCE) to V_(BL). For example, if V_(BL)is greater than V_(SOURCE), then the current may flow from the bit lineto the source. In such an example, source/drain region 430 may serve asthe drain of memory cell 408 and as the source of memory cell 410.However, if V_(BL) is less than V_(SOURCE), then the current may flowfrom the source line to the bit line. In such an example, source/drainregion 430 may serve as the source of memory cell 408 and as the drainof memory cell 410. Thus, whether a source/drain region 430 functions asthe source or as the drain of a given memory cell may depend on therelative magnitudes of V_(SOURCE) to V_(BL). As previously mentioned,the source line will be referred to herein as a source line regardlessof whether V_(SOURCE) is less than or greater than V_(BL). Moreover, thevoltage applied to the source line will be referred to herein asV_(SOURCE) regardless of whether V_(SOURCE) is less than or greater thanV_(BL).

FIG. 4 illustrates a non-volatile storage device 210 that may includeone or more memory die or chips 212. Memory die 212 includes an array(two-dimensional or three dimensional) of memory cells 200, controlcircuitry 220, and read/write circuits 230A and 230B. In one embodiment,access to the memory array 200 by the various peripheral circuits isimplemented in a symmetric fashion, on opposite sides of the array, sothat the densities of access lines and circuitry on each side arereduced by half. The read/write circuits 230A and 230B include multiplesense blocks 300 which allow a page of memory cells to be read orprogrammed in parallel. The memory array 200 is addressable by wordlines via row decoders 240A and 240B and by bit lines via columndecoders 242A and 242B. In a typical embodiment, a controller 244 isincluded in the same memory device 210 (e.g., a removable storage cardor package) as the one or more memory die 212. Commands and data aretransferred between the host and controller 244 via lines 232 andbetween the controller and the one or more memory die 212 via lines 234.One implementation can include multiple chips 212.

Control circuitry 220 cooperates with the read/write circuits 230A and230B to perform memory operations on the memory array 200. The controlcircuitry 220 includes a state machine 222, an on-chip address decoder224 and a power control module 226. The state machine 222 provideschip-level control of memory operations. The on-chip address decoder 224provides an address interface to convert between the address that isused by the host or a memory controller to the hardware address used bythe decoders 240A, 240B, 242A, and 242B. The power control module 226controls the power and voltages supplied to the word lines and bit linesduring memory operations. In one embodiment, power control module 226includes one or more charge pumps that can create voltages larger thanthe supply voltage.

In one embodiment, one or any combination of control circuitry 220,power control circuit 226, decoder circuit 224, state machine circuit222, decoder circuit 242A, decoder circuit 242B, decoder circuit 240A,decoder circuit 240B, read/write circuits 230A, read/write circuits230B, and/or controller 244 can be referred to as one or more managingcircuits.

FIG. 5 depicts an exemplary structure of memory cell array 200. In oneembodiment, the array of memory cells is divided into M blocks of memorycells. As is common for flash EEPROM systems, the block is the unit oferase. That is, each block contains the minimum number of memory cellsthat are erased together. Each block is typically divided into a numberof pages. A page is a unit of programming. One or more pages of data aretypically stored in one row of memory cells. A page can store one ormore sectors. A sector includes user data and overhead data. Overheaddata typically includes parity bits of an Error Correction Code (ECC)that have been calculated from the user data of the sector. A portion ofthe controller (described below) calculates the ECC parity when data isbeing programmed into the array, and also checks it when data is beingread from the array. Alternatively, the ECCs and/or other overhead dataare stored in different pages, or even different blocks, than the userdata to which they pertain. A sector of user data is typically 512bytes, corresponding to the size of a sector in magnetic disk drives. Alarge number of pages form a block, anywhere from 8 pages, for example,up to 32, 64, 128 or more pages. Different sized blocks and arrangementscan also be used.

In another embodiment, the bit lines are divided into odd bit lines andeven bit lines. In an odd/even bit line architecture, memory cells alonga common word line and connected to the odd bit lines are programmed atone time, while memory cells along a common word line and connected toeven bit lines are programmed at another time.

FIG. 5 also shows more details of block i of memory array 200. Block iincludes X+1 bit lines and X+1 NAND strings. Block i also includes 64data word lines (WL0-WL63), 2 dummy word lines (WL_d0 and WL_d1), adrain side select line (SGD) and a source side select line (SGS). Oneterminal of each NAND string is connected to a corresponding bit linevia a drain select gate (connected to select line SGD), and anotherterminal is connected to the source line via a source select gate(connected to select line SGS). Because there are sixty four data wordlines and two dummy word lines, each NAND string includes sixty fourdata memory cells and two dummy memory cells. In other embodiments, theNAND strings can have more or fewer than 64 data memory cells and moreor fewer dummy memory cells. Data memory cells can store user or systemdata. Dummy memory cells are typically not used to store user or systemdata. Some embodiments do not include dummy memory cells.

FIG. 6 is a block diagram of an individual sense block 300 partitionedinto a core portion, referred to as a sense module 480, and a commonportion 490. In one embodiment, there will be a separate sense module480 for each bit line and one common portion 490 for a set of multiplesense modules 480. In one example, a sense block will include one commonportion 490 and eight sense modules 480. Each of the sense modules in agroup will communicate with the associated common portion via a data bus472. For further details, refer to U.S. Patent Application Publication2006/0140007, filed Dec. 29, 2004, and titled, “Non-volatile memory andmethod with shared processing for an aggregate of read/write circuits,”which is hereby incorporated herein by reference in its entirety.

Sense module 480 comprises sense circuitry 470 that determines whether aconduction current in a connected bit line is above or below apredetermined threshold level. In some embodiments, sense module 480includes a circuit commonly referred to as a sense amplifier. Sensemodule 480 also includes a bit line latch 482 that is used to set avoltage condition on the connected bit line. For example, apredetermined state latched in bit line latch 482 will result in theconnected bit line being pulled to a state designating program inhibit(e.g., Vdd).

Common portion 490 comprises a processor 492, a set of data latches 494and an I/O Interface 496 coupled between the set of data latches 494 anddata bus 471. Processor 492 performs computations. For example, one ofits functions is to determine the data stored in the sensed memory celland store the determined data in the set of data latches. The set ofdata latches 494 is used to store data bits determined by processor 492during a read operation. It is also used to store data bits importedfrom the data bus 471 during a program operation. The imported data bitsrepresent write data meant to be programmed into the memory. I/Ointerface 496 provides an interface between data latches 494 and thedata bus 471.

During read or sensing, the operation of the system is under the controlof state machine 222 that controls the supply of different control gatevoltages to the addressed cell. As it steps through the variouspredefined control gate voltages corresponding to the various memorystates supported by the memory, the sense module 480 may trip at one ofthese voltages and an output will be provided from sense module 480 toprocessor 492 via bus 472. At that point, processor 492 determines theresultant memory state by consideration of the tripping event(s) of thesense module and the information about the applied control gate voltagefrom the state machine via input lines 493. It then computes a binaryencoding for the memory state and stores the resultant data bits intodata latches 494. In another embodiment of the core portion, bit linelatch 482 serves double duty, both as a latch for latching the output ofthe sense module 480 and also as a bit line latch as described above.

It is anticipated that some implementations will include multipleprocessors 492. In one embodiment, each processor 492 will include anoutput line (not depicted in FIG. 6) such that each of the output linesis wired-OR'd together. In some embodiments, the output lines areinverted prior to being connected to the wired-OR line. Thisconfiguration enables a quick determination during the programverification process of when the programming process has completedbecause the state machine receiving the wired-OR line can determine whenall bits being programmed have reached the desired level. For example,when each bit has reached its desired level, a logic zero for that bitwill be sent to the wired-OR line (or a data one is inverted). When allbits output a data 0 (or a data one inverted), then the state machineknows to terminate the programming process. In embodiments where eachprocessor communicates with eight sense modules, the state machine may(in some embodiments) need to read the wired-OR line eight times, orlogic is added to processor 492 to accumulate the results of theassociated bit lines such that the state machine need only read thewired-OR line one time.

During program or verify, the data to be programmed is stored in the setof data latches 494 from the data bus 471. The program operation, underthe control of the state machine, comprises a series of programmingvoltage pulses (with increasing magnitudes) applied to the control gatesof the addressed memory cells. Each programming pulse may be followed bya verify process to determine if the memory cell has been programmed tothe desired state. Processor 492 monitors the verified memory staterelative to the desired memory state. When the two are in agreement,processor 492 may set the bit line latch 482 so as to cause the bit lineto be pulled to a state designating program inhibit. This inhibits thecell coupled to the bit line from further programming even if it issubjected to programming pulses on its control gate. In otherembodiments the processor initially loads the bit line latch 482 and thesense circuitry sets it to an inhibit value during the verify process.

Data latch stack 494 contains a stack of data latches corresponding tothe sense module. In one embodiment, there are 3-5 (or another number)data latches per sense module 480. In one embodiment, the latches areeach one bit. In some implementations (but not required), the datalatches are implemented as a shift register so that the parallel datastored therein is converted to serial data for data bus 471, and viceversa. In one embodiment, all the data latches corresponding to theread/write block of m memory cells can be linked together to form ablock shift register so that a block of data can be input or output byserial transfer. In particular, the bank of read/write modules isadapted so that each of its set of data latches will shift data in to orout of the data bus in sequence as if they are part of a shift registerfor the entire read/write block.

Additional information about the read operations and sense amplifierscan be found in (1) U.S. Pat. No. 7,196,931, “Non-Volatile Memory AndMethod With Reduced Source Line Bias Errors,”; (2) U.S. Pat. No.7,023,736, “Non-Volatile Memory And Method with Improved Sensing,”; (3)U.S. Patent Application Pub. No. 2005/0169082; (4) U.S. Pat. No.7,196,928, “Compensating for Coupling During Read Operations ofNon-Volatile Memory,” and (5) United States Patent Application Pub. No.2006/0158947, “Reference Sense Amplifier For Non-Volatile Memory,”published on Jul. 20, 2006. All five of the immediately above-listedpatent documents are incorporated herein by reference in their entirety.

At the end of a successful programming process (with verification), thethreshold voltages of the memory cells should be within one or moredistributions of threshold voltages for programmed memory cells orwithin a distribution of threshold voltages for erased memory cells, asappropriate. FIG. 7A depicts example threshold voltage distributions forstates of memory cells in which there are eight states. The eight datastates include an erase state and states A-G. In this example, threebits may be stored per memory cell. Between each of the data states areread reference voltages used for reading data from memory cells. Forexample, FIG. 7A shows read reference voltage Vra between data stateserase and A, and Vrb between data states A and B. By testing whether thethreshold voltage of a given memory cell is above or below therespective read reference voltages, the system can determine what statethe memory cell is in. At or near the lower edge of each data state areverify reference voltages. For example, FIG. 7A shows VvA for state Aand VvB for state B, etc. When programming memory cells to a givenstate, the system will test whether those memory cells have a thresholdvoltage greater than or equal to the verify reference voltage.

FIG. 7B illustrates example threshold voltage distributionscorresponding to data states for the memory cell array when each memorycell stores four bits of data. Other embodiments, however, may use moreor fewer than four bits of data per memory cell. FIG. 7B shows 16threshold voltage distributions corresponding to data states 0-15. Inthe embodiment of FIG. 7B, the gap between at least some adjacent statesis the same (e.g., Δmv). For example, states 1 and 2 are relativelyclose together (separated by Δmv), whereas states 2 and 3 are furtherapart. Moreover, in this embodiment, the gap between the verify levelfor states that are close together is the same. For example, the verifylevel for state 2 is Vv2 and the verify level for state 1 is Vv2−Δ. Thisis also true for other states. In one embodiment, states that areseparated by Δ are verified together, which may save time. For example,states 0 and 1 are verified together, states 3 and 4 are verifiedtogether, etc. Note that it is not required that some adjacent states(e.g., states 2 and 3) be separated by a distance other than Δ. Alsonote that there may be a different number of data states. In someembodiments, the threshold voltages in state 0 are negative and thethreshold voltages in the states 1-15 are positive. However, thethreshold voltages in one or more of states 1-15 may be negative.

FIG. 8A depicts an example set of threshold voltage distributions for afour-state memory device in which each storage element stores two bitsof data. A first threshold voltage distribution 700 is provided forerased (Erased-state) storage elements. Three threshold voltagedistributions 702, 704 and 706 represent programmed states A, B and C,respectively. In one embodiment, the threshold voltages in theErased-state are negative, and the threshold voltages in the A-, B- andC-states are positive.

Read reference voltages, Vra, Vrb and Vrc, are also provided for readingdata from storage elements. By testing whether the threshold voltage ofa given storage element is above or below Vra, Vrb and Vrc, the systemcan determine the state, e.g., programming condition, the storageelement is in.

Further, verify reference voltages, Vva, Vvb, and Vvc are provided. Whenprogramming storage elements to the A-state, B-state or C-state, thesystem will test whether those storage elements have a threshold voltagegreater than or equal to Vva, Vvb or Vvc, respectively. In oneembodiment, “verify low” reference voltages, Vval, Vvbl, and Vvcl areprovided. Similar “verify low” reference voltages could also be used inembodiments with a different number of states.

In full sequence programming, storage elements can be programmed fromthe Erased-state directly to any of the programmed states A, B or C. Forexample, a population of storage elements to be programmed may first beerased so that all storage elements in the population are in theErased-state. A series of program pulses such as depicted in FIG. 8B isused to program storage elements directly into the A-, B- and C-states.While some storage elements are being programmed from the Erased-stateto the A-state, other storage elements are being programmed from theErased-state to the B-state and/or from the Erased-state to the C-state.Note that using a full sequence programming is not required.

One example of a slow programming mode uses low (offset) and high(target) verify levels for one or more data states. For example, VvaLand Vva are offset and target verify levels, respectively, for theA-state, and VvbL and Vvb are offset and target verify levels,respectively, for the B-state. During programming, when the thresholdvoltage of a storage element which is being programmed to the A-state asa target state (e.g., an A-state storage element) exceeds VvaL, itsprogramming speed is slowed, such as by raising Vbl to a level, e.g.,0.6-0.8 V, which is between a nominal program or non-inhibit level,e.g., 0 V and a full inhibit level, e.g., 2-3 V. This provides greateraccuracy by avoiding large step increases in threshold voltage. When thethreshold voltage reaches Vva, the storage element is locked out fromfurther programming. Similarly, when the threshold voltage of a B-statestorage element exceeds VvbL, its programming speed is slowed, and whenthe threshold voltage reaches Vvb, the storage element is locked outfrom further programming. In one approach, a slow programming mode isnot used for the highest state since some overshoot is typicallyacceptable. Instead, the slow programming mode can be used for theprogrammed states, above the erased state, and below the highest state.

Moreover, in the example programming techniques discussed, the thresholdvoltage of a storage element is raised as it is programmed to a targetdata state. However, programming techniques can be used in which thethreshold voltage of a storage element is lowered as it is programmed toa target data state. Programming techniques which measure storageelement current can be used as well. The concepts herein can be adaptedto different programming techniques.

FIG. 8B depicts a series of program and verify pulses which are appliedto a selected word line during a programming operation. A programmingoperation may include multiple programming iterations, where eachiteration applies one or more program pulses (voltages) followed by oneor more verify voltages, to a selected word line. In one possibleapproach, the program voltages are stepped up in successive iterations.Moreover, the program voltages may include a first portion which has apass voltage (Vpass) level, e.g., 6-8 V, followed by a second portion ata program level, e.g., 12-25 V. For example, first, second, third andfourth program pulses 800, 802, 804 and 806 have program voltages ofVpgm1, Vpgm2, Vpgm3 and Vpgm4, respectively, and so forth. A set of oneor more verify voltages may be provided after each program pulse. Insome embodiments, a single verify pulse is used to verify memory cellsbeing programmed to different states. For example, a single verify pulseVverify may be used to verify whether memory cells that are targeted forthe A-state have reached Vva, whether memory cells that are targeted forthe B-state have reached Vba, and whether memory cells that are targetedfor the C-state have reached Vvc. In some embodiments, there may be twoor more verify pulses between the program pulses. For example, one pulsemight be used to verify the A-state and the B-state, a second may beused to verify the C-state and a D-state, etc. In some cases, one ormore initial program pulses are not followed by verify pulses because itis not expected that any storage elements have reached the lowestprogram state (e.g., A-state). Subsequently, program iterations may useverify pulses for the A-state, followed by program iterations which useverify pulses for the A- and B-states, followed by program iterationswhich use verify pulses for the B- and C-states, for instance.

In one embodiment, different voltages are applied between the gate andsource of memory cells while reading or verifying memory cells. Forexample, in order to verify different threshold voltages during the sameverify operation different voltages are applied between the gate andsource of memory cells. As another example, in order to apply differentamounts of compensation for cross-coupling effects different voltagesare applied between the gate and source of memory cells. In oneembodiment, a memory cell on one NAND string is tested to determine ifits threshold voltage is at least at Vva at the same time that a memorycell on another NAND string is tested to determine if its thresholdvoltage is at least at Vvb. At this same time, a third memory cell on athird NAND string may be tested to determine whether its thresholdvoltage is above Vvc.

Applying Different Conditions to Different Selected Memory Cells

FIG. 9A is a schematic diagram of a circuit that is able to applydifferent gate-to-source voltages across a selected memory cell ondifferent strings of memory cells. The schematic will be used to explainprinciples of one embodiment. The voltage Vcgry may be applied to theselected word line (e.g., WL2), while Vread may be applied to unselectedword lines. The circuit has switches S1-S8 for connecting voltages VD,VD−Δ, VS, and VS−Δ to the strings of memory cells. Note that there maybe many more strings than two. Also note that in this example, thestrings of memory cells are not connected by a common source line.

Switches S2 and S5 may be closed such that VD−VS appears across onestring. Alternatively, switches 51 and S6 may be closed such that(VD−Δ)−(VS−Δ) or VD−VS appears across that string. Therefore, the Vds ofthe selected memory cell will be about the same with either switchconfiguration. However, Vgs will be Vcgry−VD for one and Vcgry−VD+Δ forthe other. Therefore, Vgs can be altered without altering Vds. SwitchesS3, S4, S7, and S8 can be used in a similar manner for the other stringof memory cells. Therefore, a different Vgs can be applied to thestrings while applying the same Vcgry to the selected word line.Consequently, two different threshold voltages can be tested for whileapplying the same Vcgry to the selected word line.

FIG. 9B is a schematic diagram of NAND strings that have differentvoltages applied across them during a read operation. This circuit maybe used when verifying different threshold voltages for different memorycells on the same word line, for applying different amounts of crosscoupling compensation to different memory cells on the same word lineduring a read or program verify, as examples. The circuit may have otheruses that involve reading two different threshold voltages at the sametime.

The NAND strings are electrically connected at one end by a commonsource line. At the other end, each NAND string is electricallyconnected to a separate bit line. The word lines that electricallyconnect gates of a row of memory cells are not shown so as to notobscure the diagram. The memory cells that are associated with theselected word line are selected for reading or verifying. That is, eachof these memory cells is to have its threshold voltage compared to somereference voltage during a verify (or read) operation. Different memorycells may have their threshold voltages compared to at least twodifferent reference voltages. For example, referring briefly to FIG. 8A,one memory cell has its threshold voltage compared to Vva, while anotherhas its threshold voltage compared to Vvb. A third memory cell mighthave its threshold voltage compared to Vvc. If, for example, this isperformed during the verify stage of programming the memory cells, theneach memory cell can be verified at the same time. For example,referring to FIG. 8B, after each program pulse, a single verify signal(Vverify) is applied to the selected word line in order to test thethreshold voltage of all the memory cells being programmed. Therefore,time can be saved in that fewer verify operations are used betweenprogram pulses. Furthermore, since fewer verify operations are required,power can be saved.

Referring again to FIG. 9B, a read reference voltage (e.g., Vcgrv) isapplied to the selected word line while a read pass voltage (e.g.,Vread) is applied to unselected word lines. Therefore, each of theselected memory cells has Vcgry applied to its gate. The read passvoltage (Vread) is a voltage level that should cause an unselectedmemory cell to turn on (e.g., to conduct a current) regardless of thestate that it is programmed to. One group of memory cells have their bitlines biased to Vs and another group have their bit lines biased toVs−Δ. For example, memory cells that are being tested for one thresholdvoltage have their bit lines biased to Vs, whereas memory cells that arebeing tested for a different threshold voltage have their bit linesbiased to Vs−Δ. Note that still other memory cells could be tested forstill another threshold voltage by applying still another voltage totheir bit lines. In this embodiment, a voltage Vsource is applied to thecommon source line. The voltage Vsource is larger than the voltagesapplied to the bit lines. Therefore, the conduction current (Icell)flows from the source line to the bit line. In effect, the bottom of thememory cells functions as the drain and the top of the memory cellsfunctions as the source. The gate, source and drain of one of theselected memory cells have been labeled. Note that a selected memorycell will turn on if its threshold voltage is equal to or less than thegate to source voltage. Also note, that as previously mentioned, forconsistency in description the line the joins the NAND strings togetherat the bottom of FIG. 9B will be referred to as a common source lineeven though the bottom of the memory cells are functioning as a drain.In one embodiment, the Vt of the cell is measured depending on both Vgsand Vds of the voltage bias. Note that if Δ is relatively small, thenVds of memory cells on the bit lines having Vs applied will be veryclose to Vds of the bit lines having Vs−Δ applied.

FIG. 9C is a schematic diagram of NAND strings that have differentvoltages applied across them during a verify operation. This circuit maybe used when verifying different threshold voltages for different memorycells on the same word line, for applying different amounts of crosscoupling compensation to different memory cells on the same word lineduring a read or program verify, as examples. The circuit may have otheruses that involve reading two different threshold voltages at the sametime. The structure is similar to the one of FIG. 9B. However, thevoltages applied to the bit lines are higher than the voltage applied tothe common source line. Therefore, the top of the selected memory cellfunctions as the drain and the bottom of the selected memory cellfunctions as the source. The conduction current flows from the bit lineto the source line in this embodiment. A read reference voltage (Vcgrv)is applied to the selected word line while a read pass voltage (Vread)is applied to unselected word lines. One of the selected memory cellshas its gate, source, and drain labeled. Note that the drain and sourceare switched compared to FIG. 9B due to the way in which voltages arebeing applied across the NAND strings.

One group of memory cells have their bit lines biased to Vd, whereasanother group of memory cell have their bit lines biased to Vd+Δ. Forexample, memory cells that are being tested for one threshold voltagehave their bit lines biased to Vd, whereas memory cells that are beingtested for a different threshold voltage have their bit lines biased toVd+Δ. Note that in this example, each memory cell on the selected wordline experiences the same gate to source voltage. That is, all selectedmemory cells have the same gate voltage and the NAND strings are joinedby the common source line. However, their drain to source voltages maybe different due to the different voltages applied to the bit lines. TheVt of the cell is measured depending on Vds of the voltage bias.

Note that a memory cell's threshold voltage may be a function of thedrain voltage. For example, the phenomena referred to as drain inducedbarrier lowering (DIBL) may alter a field effect transistor's (FET)threshold voltage. For some devices, an increase in drain voltagedecreases threshold voltage. For some devices, an increase in the drainvoltage increases the depletion region which has the effect of loweringthe threshold voltage.

In one embodiment, some bit lines have a voltage applied that causeslittle or no DIBL to the selected memory cell. Such memory cells willturn on if their threshold voltage is less than Vgs. However, other bitlines have a greater voltage applied thereto, which causes sufficientDIBL to effectively lower the threshold voltage of the selected memorycell. Therefore, those memory cells may turn on even if their actualthreshold voltage is slightly less than Vgs.

The following example will be used to illustrate. Some memory cells maybe tested to determine whether their threshold voltage is less than 3.0Vand others may be tested to determine whether their threshold voltage isless than 3.5V. A voltage of 3.0V may be applied to the selected wordline and a read pass voltage to unselected word lines. The common sourceline may be grounded. The bit lines of memory cells that are beingtested for a threshold voltage of 3.0V are biased to a voltage that isintended to cause little or no DIBL. The bit lines of memory cells thatmay be being tested for a threshold voltage of 3.5V may be biased to avoltage that is intended to cause an amount of DIBL to lower thethreshold voltage by about 0.5V, at least under the assumption thattheir threshold voltage without DIBL is near 3.5V. Therefore, the memorycells being tested for a threshold voltage of 3.5V will turn on if theirthreshold voltage is below 3.5V, but will not turn on if their thresholdvoltage is above 3.5V, given that 3.0V is applied to the selected wordline. Note that other memory cells could be tested for still anotherthreshold voltage by applying a different voltage to the bit line tocause a different shift in threshold voltage due to DIBL.

FIG. 10 is a flowchart of one embodiment of a process of sensingconditions of memory cells in different NAND strings by applyingdifferent voltages across different NAND strings. Note that in oneembodiment, the different voltages across the NAND strings cause adifferent Vgs for different memory cells being verified or read. Notethat in another embodiment, the different voltages across the NANDstrings cause a different Vds for different memory cells being verifiedor read. FIG. 10 will be discussed while referring to the examplecircuits of FIGS. 9B and 9C, as either of those circuits may be used.However, the process of FIG. 10 is not limited to those circuits.

In step 901, a suitable voltage to apply across is NAND string isdetermined. In one embodiment, this amounts to determining whether toapply Vs or Vs−Δ to a given bit line. In one embodiment, this amounts todetermining whether to apply Vd or Vd+Δ to a given bit line. Note thatthree or more different voltages could be applied across the NANDstrings. Further details of determining a suitable voltage differenceare discussed below.

In step 902, a first voltage difference is caused across a first NANDstring. For example, Vs is applied to the bit line while a voltagegreater than Vs is applied to the source line. Therefore, the conductioncurrent (if any) may flow from source line to bit line. As anotherexample, Vd is applied to the bit line while a voltage less than Vd(e.g., ground) is applied to the source line. Therefore, the conductioncurrent (if any) may flow from bit line to the source line. In oneembodiment, the first NAND string has a memory cell that is beingverified at a first threshold level. For example, a determination isbeing made whether its threshold voltage is greater than Vva.

In 904, a second voltage difference is caused across a second NANDstring. For example, Vs−Δ is applied to the bit line while a voltagegreater than Vs−Δ is applied to the source line. Since the source lineis common for all bit lines, the source line voltage will be same as instep 902. As another example, Vd+Δ is applied to the bit line while avoltage less than Vd+Δ (e.g., ground) is applied to the source line. Inone embodiment, the second NAND string has a memory cell that is beingverified at a second threshold level. For example, a determination isbeing made whether its threshold voltage is greater than Vvb.

In optional step 906, a third voltage difference is caused across athird NAND string. For example, Vs−2Δ is applied to the bit line while avoltage greater than Vs−2Δ is applied to the source line. As anotherexample, Vd+2Δ is applied to the bit line while a voltage less thanVd+2Δ (e.g., ground) is applied to the source line. In one embodiment,the third NAND string has a memory cell that is being verified at athird threshold level. For example, a determination is being madewhether its threshold voltage is greater than Vvc.

Steps 902, 904, and 906 may be performed together. Note that there maybe thousands of selected memory cells along the selected word line.Therefore, step 902 may be applied to many different NAND strings at thesame time. Likewise, steps 904 and 906 (if performed) may be performedon many different NAND strings.

In step 908, a read reference voltage is applied to a selected word linewhile causing the first and second voltage differences. For example,Vcgry may be applied to the selected word line while Vsource is appliedto the common source line and either Vs or Vs−Δ is applied to the bitline. Alternatively, Vcgry may be applied to the selected word linewhile Vsource is applied to the common source line and either Vd or Vd−Δis applied to the bit line. Therefore, a single reference voltage may beapplied to the gates of the selected memory cells.

In step 910, a first condition of the first non-volatile storage elementis sensed in response to the first voltage difference and read voltageto determine whether the threshold voltage of the first non-volatilestorage element is above or below a first reference voltage. Forexample, the conduction current of a memory cell is sensed to determinewhether the threshold voltage is greater than Vva.

In step 912, a second condition of the second non-volatile storageelement is sensed in response to the second voltage difference and readvoltage to determine whether the threshold voltage of the secondnon-volatile storage element is above or below a second referencevoltage. The second reference voltage may be different from the firstreference voltage. For example, the conduction current of a memory cellis sensed to determine whether the threshold voltage is greater thanVvb. Note that it is not required that the first and second referencevoltages are verify levels. For example, the first reference voltagemight be Vva+Δ and the second reference voltage might be Vva−Δ. Alsonote that it is not required that the first and second referencevoltages be different. For example, the process of FIG. 10 may be usedto apply different amounts of cross-coupling compensation. In this case,the first and second reference voltages may be the same. For example,the process could test for whether the threshold voltage of twodifferent memory cells is greater than Vva while applying a differentamount of cross-coupling compensation to each the memory cells.

In optional step 914, a third condition of the third non-volatilestorage element is sensed in response to the third voltage differenceand read voltage to determine whether the threshold voltage of the thirdnon-volatile storage element is above or below a third referencevoltage. In one embodiment, the third reference voltage is differentfrom both the first and second reference voltages. For example, theconduction current of a memory cell is sensed to determine whether thethreshold voltage is greater than Vvc.

FIG. 11A is a block diagram of one embodiment of a sense module 480 forcharging a bit line to a voltage that is lower than the source linevoltage. The sense module 480 may be used when implementing any of steps902, 904, or 906 from FIG. 10. For example, the sense module 480 may beused to establish a certain voltage on the bit line. Another circuit(not depicted in FIG. 11A) may establish a certain voltage on the sourceline. Therefore, the sense module 480 may be used to create a certainvoltage difference across a NAND string. The voltage on the bit line maybe less than the source line voltage. The sense module 480 could be usedto establish Vs and Vs−Δ for the example circuit of FIG. 9B.

The sense module 480 has a current sink 1130, current source 1230,current sense 1132, and control circuitry 1134. Current sink 1130 may beused for reverse sensing and may include 1 to n different bit linevoltage setups to apply different voltages to the bit line. Currentsource 1230 may be used for forward sensing and may also include 1 to ndifferent voltage setups to apply different voltages to the bit line.The control circuit 1134 controls switch S11 connects the current sink1130 to the bit line. The control circuit 1134 controls switch S13connects the current source 1230 to the bit line. The control circuit1134 controls switch S12 to connect the current sense 1132 to the bitline. In one embodiment, the control circuitry 1134 connects the currentsink 1130 to the bit line to sink IchargeB to reduce the voltage to atarget voltage. The current sense is one implementation of sensecircuitry 470 of the sense module of FIG. 6. The circuit of FIG. 24provides further details. Other implementations are possible. FIG. 11Bis a block diagram the sense module 480 of FIG. 11A showing thedirection of current when sensing a memory cell's threshold voltage.

The control circuit 1134 receives data, which is used to determine whatvoltage level the bit line should be charged to. In one embodiment, thedata is based on the reference level is being verified. For example, thedata might indicate whether Vva, Vvb, Vvc or another reference level isbeing verified. In one embodiment, the data is based on the amount ofcross-coupling compensation that is desired. For example, the data mightindicate whether a low or high amount of compensation is desired. In oneembodiment, the cross-coupling compensation is based on a neighbormemory cell's Vt. In one embodiment, the cross-coupling compensation isbased on the state to which neighbor memory cell is being programmed.Therefore, the data may indicated the neighbor's actual or intended datastate, which based on the Vt. Further details are discussed below. Inone embodiment, the data is based on a previous reading of the memorycell being read. For example, the data could indicate whether the memorycell was previously determined to have a threshold voltage above/below atarget Vt.

FIG. 12A is a block diagram of one embodiment of a sense module 480 forcharging a bit line to a voltage that is higher than the source linevoltage. The sense module 480 may be used when implementing any of steps902, 904, or 906 from FIG. 10. For example, the sense module 480 may beused to create a certain voltage difference across a NAND string. Thesense module 480 has a current source 1230, and current sense 1132.Switch S21 connects the current source 1230 to the bit line. Switch S22connects the current sense 1132 to the bit line. FIG. 12B is a blockdiagram the sense module 480 of FIG. 12A showing the direction ofcurrent when sensing a memory cell's threshold voltage. As with thecircuit of FIG. 11A, the control circuit 1134 receives data, which isused to determine to which voltage level the bit line should be charged.The sense module 480 could be used to establish Vd and Vd+Δ for theexample circuit of FIG. 9C.

FIG. 13A is a flowchart of one embodiment of a process of determining asuitable voltage to apply across a NAND string based on the state beingverified. The process is one embodiment of step 901 of FIG. 10. Thisprocess can be performed for every memory cell that is being verified.In step 1202, a determination is made as to which state is beingverified. In one embodiment, a latch that is connected to the sensemodule 480 holds data that indicates what state the selected memory cellis being programmed to. This latch may maintain this value at least aslong as the memory cell is being programmed such that its value can beused to determine the verify level. In one embodiment, the data fromthis latch is the data that is input to control circuitry 1134 of eitherFIG. 11A or 12A.

In step 1204, a suitable voltage is determined to apply to the bit linebased on the state being verified. In one embodiment, the controlcircuitry 1134 determines the appropriate voltage to apply to the bitline. Note that in this embodiment it is assumed that all memory cellsare connected by a common source line. Therefore, determining a voltageto apply to the bit line, in effect, determines a voltage to applyacross the NAND strings. Further details of one embodiment ofdetermining and controlling the bit line voltage are described below.

FIG. 13B is a flowchart of one embodiment of a process of determining asuitable voltage to apply across a NAND string to compensate forcross-coupling during read. The process is one embodiment of step 901 ofFIG. 10. In this embodiment, the determination is based on the thresholdvoltage of a neighbor memory cell. This process can be performed forevery memory cell that is being read. In step 1212, a determination ismade as to the threshold voltage or state of the neighbor memory cell.In one embodiment, the neighbor memory cell is read to gain someinformation about its Vt. It is not required to know the exact state ofthe neighbor memory cell. Further details are discussed below. In oneembodiment, some indication of the neighbor's threshold voltage is thedata that is input to control circuitry 1134 of either FIG. 11A or 12A.

In step 1214, a suitable voltage is determined to apply to the bit lineto compensate for cross-coupling due to the neighbor's Vt. In oneembodiment, the control circuitry 1134 determines the appropriatevoltage to apply to the bit line. For example, the input data might bewhether the neighbor's threshold voltage is “high” or “low.” If it ishigh, then a first voltage is to be applied to the bit line to achieve afirst amount of compensation for cross-coupling. If it is low, then asecond voltage is to be applied to the bit line to achieve a secondamount of compensation for cross-coupling.

FIG. 13C is a flowchart of one embodiment of a process of determining asuitable voltage to apply across a NAND string to compensate forcross-coupling during programming. The process is one embodiment of step901 of FIG. 10. In this embodiment, the determination is based on thethreshold voltage to which a neighbor memory cell will be programmed.This process can be performed for every memory cell that is beingprogrammed. In step 1224, a determination is made as to the state towhich the neighbor memory cell will be programmed. In one embodiment,there is a latch that holds data that indicates the intended state towhich the neighbor will be programmed. In one embodiment, the data fromthis latch is the data that is input to control circuitry 1134 of eitherFIG. 11A or 12A.

In step 1224, a suitable voltage is determined to apply to the bit lineto compensate for cross-coupling due to the later programming of theneighbor. In one embodiment, the control circuitry 1134 determines theappropriate voltage to apply to the bit line. For example, the inputdata might be whether the neighbor's threshold voltage will be “high” or“low.” If it will be high, then a first voltage is to be applied to thebit line to achieve a first amount of compensation for cross-coupling.If the neighbor's threshold voltage will be will be low, then a secondvoltage is to be applied to the bit line to achieve a second amount ofcompensation for cross-coupling.

FIG. 13D is a flowchart of one embodiment of a process of determining asuitable voltage to apply across a NAND string based on a previousdetermination relating to the threshold voltage of the memory cell. Thismay be used to perform a binary search to locate the threshold voltagewith any desired accuracy. Performing a binary search will be discussedmore fully below. The process of FIG. 13D is one embodiment of step 901of FIG. 10. This process can be performed for every memory cell that isbeing read. In step 1232, information regarding a previous reading ofthe threshold voltage of the selected memory cell is accessed. Forexample, a latch may hold a value that indicates the result of aprevious comparison of the conduction current of the selected memorycell with a reference current. If the reference current is selected totest for a target Vt, then the value in the latch indicates whether theselected memory cell's threshold voltage is above or below a target Vt.In one embodiment, the data from this latch is the data that is input tocontrol circuitry 1134 of either FIG. 11A or 12A.

In step 1234, a suitable voltage to apply to the bit line is determinedbased on the previous reading for the Vt. For example, assume that theprevious reading determined whether the threshold voltage was above orbelow a target Vt. One voltage to apply to the bit line might be able totest for the target threshold voltage+Δ. Another voltage to apply to thebit line might be able to test for the target threshold voltage−Δ. Ifthe previous reading indicated that the threshold voltage was below thetarget, then a voltage is applied to test for the target thresholdvoltage−Δ. If the previous reading indicated that the threshold voltagewas above the target, then a voltage is applied to test for the targetthreshold voltage+Δ.

FIG. 14A is a flowchart of one embodiment of a process for establishingdifferent voltages across NAND strings. FIG. 14A is one implementationof steps 902-904 of FIG. 10. FIG. 14A is an embodiment in whichdifferent Vgs are used for different NAND strings. For example,different threshold voltages could be sensed by applying different Vgsto different memory cells. In one embodiment, different Vgs and Vds areapplied to sense different threshold voltages. Alternatively, differentamounts of cross-coupling could be provided by applying different Vgs todifferent memory cells. In one embodiment, different Vgs and Vds areapplied to provide different amounts of cross-coupling. The circuitry ofFIG. 11A may be used when implementing the process of FIG. 14.

In step 1402, a voltage is applied to a common source line. For example,referring to FIG. 11A, the voltage Vsource is applied to the sourceline. An example, level for Vsource is 2.5V. Note that Vdd may be alower voltage such as 1.6V.

In step 1404, Vs is applied to bit lines of a first group of memorycells. The first group may include memory cells whose threshold voltageis being compared to a first voltage level. The first group may includememory cells for which a first amount of cross-coupling compensation isdesired. As an example, Vs might be 2.1V if Vsource is 2.5V. Toestablish Vs, the control circuitry 1134 may close first close switchS13 to connect the current source 1230 to the bit line. Then, thecontrol circuitry 1134 closes switch S11 to connect the current sink1130 to the bit line to achieve the target voltage. The controlcircuitry 1134 may determine when the bit line is at Vs and maydisconnect the current sink 1130 from the bit line to hold the bit lineat Vs.

In step 1406, Vs−Δ is applied to bit lines associated with a secondgroup of memory cells. The second group may include memory cells whosethreshold voltage is being compared to a second voltage level. Thesecond group may include memory cells for which a second amount ofcross-coupling compensation is desired. As an example, Vs−Δ might be2.1V if Vsource is 2.5V. To establish Vs−Δ, the control circuitry 1134may close first close switch S13 to connect the current source 1230 tothe bit line. Then, the control circuitry 1134 closes switch S11 toconnect the current sink 1130 to the bit line to achieve the targetvoltage. The control circuitry 1134 may determine when the bit line isat Vs−Δ and may disconnect the current sink 1130 from the bit line tohold the bit line at Vs−Δ. Note that steps 1404 and 1406 may beperformed together.

FIG. 14B is a flowchart of one embodiment of a process for establishingdifferent voltages across NAND strings. FIG. 14B is one implementationof steps 902-904 of FIG. 10. FIG. 14B is an embodiment in which adifferent Vds is used for different NAND strings. For example, differentthreshold voltages could be sensed. Alternatively, different amounts ofcross-coupling could be provided. The process of FIG. 14B may use thecircuit of FIG. 12A, although that is not required. The process of FIG.14B may be performed simultaneously on all NAND strings having a memorycell being verified or read. In one embodiment, the different Vds causesa different amount of DIBL.

In step 1412, a voltage is applied to a common source line. For example,referring to FIG. 11A, the voltage Vsource is applied to the sourceline. An example, level for Vsource is 1.2V. Note that Vdd may be ahigher voltage such as 2.5V.

In step 1414, Vd is applied to bit lines of a first group of memorycells. The first group may include memory cells whose threshold voltageis being compared to a first voltage level. The first group may includememory cells for which a first amount of cross-coupling compensation isdesired. As an example, Vd might be 1.6V if Vsource is 1.2V. In thisexample, Vds will be about 0.4V. To establish Vd, the control circuitry1134 may close switch S21 to connect the current source 1230 to the bitline. The control circuitry 1134 may determine when the bit line is atVd and may disconnect the current source 1230 from the bit line to holdthe bit line at Vd.

In step 1416, Vd+Δ is applied to bit lines associated with a secondgroup of memory cells. The second group may include memory cells whosethreshold voltage is being compared to a second voltage level. Thesecond group may include memory cells for which a second amount ofcross-coupling compensation is desired. The voltage Vd+Δ causes asufficient amount of DIBL to alter (e.g., lower) the threshold voltageof the memory cells by the difference between the first and secondvoltage levels. As an example, Vd+Δ may be about 2.0V with Vsource being1.2V. Therefore, Vds may be about 0.8V. To establish Vd+Δ, the controlcircuitry 1134 may close switch S21 to connect the current source 1230to the bit line. The control circuitry 1134 may determine when the bitline is at Vd+Δ and may disconnect the current source 1230 from the bitline to hold the bit line at Vd+Δ. Note that steps 1414 and 1416 may beperformed together.

FIG. 15 is a flowchart describing one embodiment of a programmingprocess, which includes one or more verification steps. Embodimentsdisclosed herein can speed up the programming by verifying multiplestates during a single verification operation. In step 1502, the portionof the memory to be programmed is selected. In one implementation, thiscan be one or more write units appropriate to the memory structure. Oneexample of a write unit is referred to as a page. In other embodiments,other units and/or structures can also be used. In step 1504, apre-programming process is sometimes used wherein the addressed memorycells are given non-data dependent programming to level out storageelement wear and provide a more uniform starting point for thesubsequent erase. In step 1506, an erase process is performed, asappropriate for the type of storage element being used. One example of asuitable smart erase process is described in U.S. Pat. No. 5,095,344,incorporated herein by reference in its entirety. Step 1508 includes asoft programming process designed to put the threshold voltages oferased memory cells into a more uniform starting range for the actualwrite phase. In one embodiment, if any of the memory cells fail toverify during erase (or during soft programming), they can be mapped outof the logical address space. At this point the memory is ready for thedata conditional programming phase.

In step 1510, the program voltage (Vpgm) is set to an initial value. Forexample, in some embodiments, the staircase wave form of FIG. 7 b isused and step 1510 includes setting the initial pulse. Also, in step1510, a program counter (PC) is initialized to zero. In step 1520, aprogram pulse is applied. For example, one iteration of step 1520 couldinclude applying program pulse 800 of FIG. 8B.

In step 1522, a verification process is performed. The verificationprocess may verify more than one state at a time. Referring to FIG. 8A,in one embodiment, some memory cells that are being programmed to theA-state are verified for the Vva level, while other memory cells thatare being programmed to the B-state are verified for the VvB level. Inone embodiment, the verification is a concurrent coarse/fine verify.Referring to FIG. 8A, some memory cells that are being programmed to theA-state are verified for the VvaL level, while others that are beingprogrammed to the A-state are verified for the Vva level. During theinitial programming steps in which the memory cell's threshold is wellbelow the final level (Vva), course programming is applied. However,after the memory cell's threshold voltage reaches VvaL, fine programmingis used. Thus, while some memory cells are being verified for coarseprogramming, other memory cells are being verified for fine programming.Note that with course/fine programming, some memory cells are beingverified for one state (e.g., A-state), while others are being verifiedfor another state (e.g., B-state). Note that when a particular memorycell has been verified as being programmed to its intended state, it maybe locked out from further programming. More details of step 1522 aredescribed below. In some embodiments, VvaL and Vva (see, for example,FIG. 8 a) can be sensed at the same time. Likewise, VvbL and Vvb, etc.may be sensed together.

In step 1524, it is determined whether all of the memory cells haveverified that their threshold voltages are at the final target voltagefor that memory cell. If so, the programming process is completedsuccessfully (status=pass) in step 1526. If all of the memory cells arenot all verified, then it is determined whether the program counter (PC)is less than a maximum value such as 20. If the program counter (PC) isnot less than max (step 1528), then the program process has failed (step1530). If the program counter (PC) is less than a maximum value (e.g.,20), then the program counter (PC) is incremented by 1 and the programvoltage is stepped up to the next pulse in step 1530. Subsequent to step1530, the process loops back to step 1520 and the next program pulse isapplied to the memory cells.

FIG. 16A is a flowchart of one embodiment of a process of verifyingmemory cells during a programming operation. In this process reversesensing is used. The process is one implementation of step 1522 of theprogramming process of FIG. 15. Therefore, the process may be appliedonce during each iteration of the programming process. FIG. 16B is atiming diagram showing one embodiment of voltages applied to word lineand bit lines during the verify process of FIG. 16A. Note that thevoltage applied to the selected word line is modified during theprocess. For example, the voltage is raised to level “A/B” in order toverify the A-state and the B-state, then the voltage is raised to level“C/D” in order to verify the C-state and the D-state, etc. Furtherdetails are discussed below.

In step 1602, the common source line is biased to Vsource. As anexample, the source line is biased to 2.5V.

In step 1604, first and second bit lines are biased to differentvoltages. The first group contains bit lines that are to have Vs appliedto the bit line. The second group contains bit lines that are to haveVs−Δ applied to the bit line. There may also be a third group thatcontains bit lines that are to have Vs applied. For example, bit linesfor memory cells being programmed to the A-, C-, E-, and G-state may beput into the first group. Bit lines for memory cells being programmed tothe B-, D-, and F-state may be put into the second group. Bit lines formemory cells that are to stay erased may be put into the third group. Inone embodiment, one of the bit lines from the first group (e.g., theA-state) and one of the bit lines from the second group (e.g., theB-state) may be being verified at a time. During step 1604, all the bitlines in the first group may be biased to Vs, all of the bit lines inthe second group may be biased to Vs−Δ, and all of the bit lines in thethird group may be biased to Vsource.

In step 1606, voltages are applied to the source side select gate (SGS)and the drain side select gate (SGD). As an example, SGS and SGD arebiased to Vsource+Vt, where threshold voltage is the threshold voltageof the SGS transistor.

In step 1608, Vread is applied to unselected word lines. In step 1610, asuitable read voltage is applied to the selected word line. Referring toFIG. 16B, the voltage on the selected word line may be raised to thelevel labeled “A/B” in order to verify the A-state and the B-state. Thislevel will create a Vgs that is suitable to verify the memory cellsbeing programmed to the A-state, as well as a Vgs that is suitable toverify the memory cells being programmed to the B-state.

In step 1612, the bit lines of memory cells to be verified are sensed.If the threshold voltage of a memory cell being verified is below itstarget state it should turn on and conduct a sizeable current, which maybe detected. For example, the current sense 1132 of FIG. 11B maydetermine whether or not the memory cell conducts a sizeable currentbased on Icell. Note that other sensing techniques could be used.

If there are more states to verify (step 1614), then the process returnsto step 1610. In step 1610, the voltage on the selected word line isagain set to the level that is appropriate for the states beingverified. For example, the voltage may be changed to the level labeled“C/D” in FIG. 16B to verify memory cells being programmed to the C-stateand the D-state. This level on the selected word line will create a Vgsthat is suitable to verify the memory cells being programmed to theC-state, as well as a Vgs that is suitable to verify the memory cellsbeing programmed to the D-state. The cells targeted to C-state may havedifferent bitline bias compared with the cells targeted to D states.

When all states are verified, the process completes. Note that it is notan absolute requirement that all states be verified. For example, earlyin the programming process it might not be necessary to verify thehighest states. Therefore, the process could end without raising theselected word line voltage to the levels “E/F” and “G.” Likewise, laterin the programming process it might not be necessary to verify thelowest states. Therefore, the process could skip raising the selectedword line voltage to the levels “A/B.” Note that for any memory cellthat is verified to have reached its intended state, programming may belocked out such that it is not affected by further programming pulses.

Note that while FIG. 16A was described in connection with the examplewaveforms of FIG. 16B, the process is not limited to those waveforms.For example, there could be more or fewer than eight states. Also,instead of verifying two states at a time, three or more states could beverified with each iteration of the process.

Also note that while step 1604 contemplates biasing bit lines that areverified by applying different voltages to the selected word line, thisis not required. For example, when verifying the A-state and theB-state, it is not required that the bit lines for the other states arebiased to Vs and Vs+Δ. Instead, those bit lines could be biased toVsource. Then, after verifying the A-state and the B-state, a differentset of bias conditions could be applied to the bit lines to verify theC-state and the D-state.

FIG. 17A is a flowchart of one embodiment of a process of verifyingmemory cells during a programming operation. This is one implementationof reverse sensing. The process is one implementation of step 1522 ofthe programming process of FIG. 15. Therefore, the process may beapplied once during each iteration of the programming process. FIG. 17Bis a timing diagram showing one embodiment of voltages applied to wordline and bit lines during the verify process of FIG. 17A. Referring toFIG. 17B, note that to verify different states, the voltage applied toSGS is changed from the level labeled “C”, then to “B”, then to “A” inorder to verify the C-state, then the B-state, then the A-state. Otherlevels could be used to verify other states. This process different fromthe one of FIG. 16A in that rather than altering the voltage on theselected word line, the voltage on SGS is altered.

FIG. 16A described simultaneous verify of only two states each time (outof e.g., eight states). A reason for simultaneously verifying only twostates and not four states or all eight states is for reducing the riskof cell punch-through, which may occur due to large bias conditiondifferences that may result in large Vds voltage difference over thecell. In one embodiment, the drain side voltage is controlled in orderto overcome the cell punch-through problem, which may allowsimultaneously verifying more states. FIG. 17A is a flowchart of oneembodiment of a process that controls the drain side voltage to avoidcell punch-through.

When discussing FIGS. 17A and 17B, reference will be made to FIGS. 18A,18B, and 18C. FIG. 18A is a diagram of several NAND strings showingexample voltages applied when verifying the C-state in the process ofFIG. 17A. FIG. 18B is a diagram of several NAND strings showing examplevoltages applied when verifying the B-state in the process of FIG. 17A.FIG. 18C is a diagram of several NAND strings showing example voltagesapplied when verifying the A-state in the process of FIG. 17A.

In the process of FIG. 17A, the voltage applied to the source sideselect line (SGS) is used to control the drain side voltage to avoid thepunch-through. In this case, all states (e.g., A,B,C in the example ofFIG. 17B) may be verified simultaneously. Therefore, only a singlevoltage is applied to the selected WL (see FIG. 17B). All states may beverified almost simultaneously because SGS is ramped up gradually.Therefore, this method may be referred to as “pseudo-simultaneous multistate verify”. Note that for some memory devices, SGS may have a shorterRC delay than that of the word line; therefore, voltage changes to SGSwill be faster than voltages changes to the word line.

In step 1702, the common source line is biased to Vsource. As anexample, the source line is biased to 2.5V. In step 1704, bit lines arebiased at an appropriate voltage for the states to be verified. Forexample, bit lines for the erase state, A-state, B-state, and C-stateeach receive a different bias. In one embodiment, the bit lines for theerase state receive the highest bias, A-state the next highest bias,B-state the next highest bias, and C-state receive the lowest bias. Forexample, bit lines for the erase state are biased to 2.5V, bit lines forthe A-state are biased to 2.0V, bit lines for the B-state are biased to1.5V, and bit lines for the C-state are biased to 1.0V.

In one embodiment, the bit line verify is performed on an even bitline/odd bit line basis. For example, during one performance of processof FIG. 17A only the odd bit lines are verified. In this case, the evenbit lines may have the same voltage as the bit lines having memory cellsthat are to stay erased. For example, Vsource (e.g., 2.5) may be appliedto these “unselected” bit lines. After verifying memory cells on the oddbit lines, the process of FIG. 17A may then be repeated to verify memorycells on the even bit lines.

In step 1706, a voltage is applied to the drain side select gate (SGD).As an example, SGD is biased to Vsource+Vt, where threshold voltage isthe threshold voltage of the SGD transistor. Note that at this time, SGSis held at a relatively low value such as ground. This low valueprevents the NAND strings from conducting a current.

In step 1708, Vread may be applied to unselected word lines. In step1710, a suitable read (or verify) voltage is applied to the selectedword line. The read voltage should be sufficient such to verify thememory cells being verified for the C-state. For example, if thethreshold voltage for the C-state is 3.0V, then 4.0V may be applied tothe selected word line. Recall that 1.0V was applied to the bit linesfor memory cells being programmed to the C-state in this example.Therefore, Vgs for the C-state memory cells will be 3.0V. Note that Vgsfor memory cells being verified for the A-state and B-state may be highenough for those memory cells to turn on. However, because the signalapplied to SGS is low at this time, none of the NAND strings conduct acurrent. Referring to FIG. 17B, note that the voltage applied to theselected word line is not required to change in order to verifydifferent states.

In step 1712, the source side select gate is raised to a level that issufficient to allow NAND strings having a memory cell being verified forthe C-state to conduct. Referring to FIG. 17B, SGS is raised to level“C”. As an example, level C may be 1.5V+Vt, where threshold voltage isthe threshold voltage of the SGS transistor. Raising SGS to level Callows NAND strings that have a memory cell being verified for theC-state to conduct. However, NAND strings that have a memory cell beingverified for either the B- or A-state will not conduct a current.Referring to FIG. 18A, with SGS at 1.5V+threshold voltage (of the SGStransistor), the source of the SGS transistor that is lightly on shouldbe about 1.5V. Therefore, note that the bottom of the NAND stringlabeled C-state is at 1.5V and the top is at 1.0V. Therefore, Vds forthe memory cell is about 0.5V. However, note that for the NAND stringlabeled B-state that there is 1.5V at each end of the NAND string.Therefore, the memory cell on the selected word line should not conducta strong current even if its threshold voltage is below Vgs.

In step 1714, the bit lines of memory cells being programmed to theC-state are sensed. If the threshold voltage of a memory cell beingverified is below its target state it should turn on and conduct asizeable current, which may be detected. For example, the current sense1132 of FIG. 11B may determine whether or not the memory cell conducts asizeable current based on Icell. Note that other sensing techniquescould be used.

In step 1715, the voltage of the bit lines of memory cells beingverified for the C-state may be raised. For example, the bit linevoltage is raised to 2.5V. This may help to prevent punch-throughconduction during later steps of the process. As another alternative,the bit lines may be floated.

In step 1716, the voltage applied to SGS is raised to a level that issufficient to allow conduction of NAND strings having a memory cellbeing verified for the B-state. Referring to FIG. 17B, SGS is raised tolevel “B.” As an example, the voltage applied to SGS is raised to2.0V+Vt, where threshold voltage is the threshold voltage of the SGStransistors. Referring to FIG. 18B, the voltage at the source of the SGStransistors on the NAND strings labeled A-, B-, and C-state are all atabout 2.0V. This should not be sufficient to allow conduction of currenton the NAND string labeled A-state, as 2.0V is also applied to the bitline. However, the NAND strings labeled B-state should be able toconduct a current. Note that because the bit lines of memory cells beingprogrammed to the C-state were raised to 2.5V (or have been floated),there should not be a conduction current for those NAND strings.

In step 1718, the bit lines of memory cells being programmed to theB-state are sensed. If the threshold voltage of a memory cell beingverified is below its target state it should turn on and conduct asizeable current, which may be detected. For example, the current sense1132 of FIG. 11B may determine whether or not the memory cell conducts asizeable current based on Icell. Note that other sensing techniquescould be used.

In step 1719, the voltage of the bit lines of memory cells beingverified for the B-state may be raised. For example, the bit linevoltage is raised to 2.5V. This may help to prevent punch-throughconduction during later steps of the process. As another alternative,the bit lines may be floated.

In step 1720, the voltage applied to SGS is raised to a level that issufficient to allow conduction of NAND strings having a memory cellbeing verified for the A-state. Referring to FIG. 17B, SGS is raised tolevel “A.” As an example, the voltage applied to SGS is raised to2.5V+Vt, where Vt is the threshold voltage of the SGS transistors.Referring to FIG. 18C, the voltage at the source of the SGS transistorson the NAND strings labeled A-, B-, and C-state are all at about 2.5V.This should be sufficient to allow conduction of current on the NANDstring labeled A-state, as only 2.0V is applied to its NAND string.However, because the bit lines of memory cells being programmed to theB- and C-states were raised to 2.5V (or have been floated), there shouldnot be a conduction current for those NAND strings.

In step 1722, the bit lines of memory cells being programmed to theA-state are sensed. If the threshold voltage of a memory cell beingverified is below its target state it should turn on and conduct asizeable current, which may be detected. The possible conduction current(Icell) is shown as a dashed line. For example, the current sense 1132of FIG. 11B may determine whether or not the memory cell conducts asizeable current based on Icell. Note that other sensing techniquescould be used. At this point all states are verified and processcompletes. Note that for any memory cell that is verified to havereached its intended state, programming may be locked out such that itis not affected by further programming pulses.

FIG. 19 is a diagram of one embodiment of a NAND string that has atransistor to help control Vds when performing reverse sensing. Thecircuit of FIG. 19 may reduce or eliminate punch-through conduction whenverifying different reference levels at the same time. The circuit ofFIG. 19 may be used with the process of FIG. 16A and the timing diagramsof FIG. 16B. For example, in FIG. 16B the voltages to SGS and SGS areramped up once during sensing of all of the states. This is in contrastto the technique depicted in FIGS. 17A and 17B in which SGS is ramped todifferent voltages during sensing of different states. In someembodiments, the circuit of FIG. 19 is used to simultaneously verifymore than two states without risk of punch-through conduction. In oneembodiment, all states are simultaneously verified.

The NAND string in FIG. 19 includes a depletion type NMOS transistorwith its gate connected to the SGD transistor. Specifically, the gate ofthe depletion type NMOS transistor is connected to the drain of the SGDtransistor. In this example, Vs is applied to the bit line and 2.5V isapplied to the source line. Transistor SGD has 2.5V+Vtsgd applied to itsgate (Vtsgd is the threshold voltage of transistor SGD). Transistor SGShas 2.5V+Vtsgs applied to its gate (Vtsgs is the threshold voltage oftransistor SGS). This may result in node A being about Vs and node Bbeing about Vs−Vth NMOS, where Vth NMOS is the threshold voltage of theNMOS transistor. If the threshold voltage of the NMOS transistor isabout −0.5V, then node B will be about Vs+0.5V. Therefore, the voltageacross the NAND string will be about 0.5V above the voltage of the bitline. Note that if the voltage of the bit line where Vs+Δ, then thevoltage at node B would be about Vs+Δ+0.5V. Again, the voltage acrossthe NAND string will be about 0.5V. Also note that this means that Vdsof the memory cell that is selected for verify may be about 0.5V.Keeping Vds at a relatively low value may prevent punch throughconduction.

Applying Different Amounts of Cross-Coupling Compensation

Sensing schemes disclosed herein for verifying multiple states can beused for cross-coupling compensation during programming or reading.Cross-coupling compensation can be used to compensate for shifts in theapparent charge stored on a floating gate which can occur because of thecoupling of an electric field based on the charge stored in adjacentfloating gates. This floating gate to floating gate coupling phenomenais described in U.S. Pat. No. 5,867,429, which is incorporated herein byreference in its entirety. The floating gate to floating gate couplingphenomena occurs most pronouncedly between sets of adjacent memory cellsthat have been programmed at different times. For example, a firstmemory cell is programmed to add a level of charge to its floating gatethat corresponds to one set of data. Subsequently, one or more adjacentmemory cells are programmed to add a level of charge to their floatinggates that correspond to a second set of data. After the one or more ofthe adjacent memory cells are programmed, the charge level read from thefirst memory cell appears to be different than programmed because of theeffect of the charge on the adjacent memory cells being coupled to thefirst memory cell. The coupling from adjacent memory cells can shift theapparent charge level being read a sufficient amount to lead to anerroneous reading of the data stored.

In one embodiment, during a read operation, the bit line of each memorycell along the selected word line is biased according to the read Vt ofthe memory cell on a neighboring word line or according to the estimatedstate of the neighboring cell given its read Vt. Note that the actualstate to which the neighboring cell was programmed may not be known.Also, the induced cross-coupling by the neighboring cell may be afunction of its present threshold voltage and not of its programmedstate. In one embodiment, during programming, the bit line of eachmemory cell along the selected word line is biased according to theintended state of the memory cell on the neighboring word line.

FIG. 20 depicts a flowchart of one embodiment of a process of applyingcross-coupling compensation during reading. In general, the processcompensates for the effects that the state of the memory cell on aneighboring word line has when reading a selected memory cell. Theprocess may employ sensing techniques that apply different voltages todifferent bit lines. For example, the process of FIG. 10, 14A, or 14Bmay be used. In some embodiments, reverse sensing is used. For example,the memory cell current may flow in the direction depicted in FIG. 9B.In some embodiments, forward sensing is used. For example, the memorycell current may flow in the direction depicted in FIG. 9C.

In the following description the term “target word line” refers to theword line having memory cells that are ultimately to be read. The targetword line may also be referred to as WLn. In step 2002, the memory cellson a word line that neighbors the target word line are read to determineat least a high threshold voltage group and a low threshold voltagegroup. For example, the memory cells on WLn+1 are read to determine atleast a high threshold voltage group and a low threshold voltage group.For example, memory cells in the erased state or A-state may beconsidered low threshold voltage group and those in either the B-stateor C-state may be considered to be high threshold voltage group. It isnot required that the exact state of each memory cell be determined. Forexample, a single read may be performed to determine whether thethreshold voltage is above or below Vrb (see FIG. 8A). Note that thememory cells may be programmed to more than four states. Also note thatif desired the memory cells could be placed into more than two groupsbased on their threshold voltages. For example, there may be a high,medium, and low threshold voltage group.

In steps 2004 and 2006, appropriate voltages are applied to bit lines tocompensate for the effect of the neighbor memory cell's thresholdvoltage when the selected memory cell is read. For example, if theneighbor memory cell is in the low threshold voltage group, then Vsmight be applied to the bit line. On the other hand, if the neighbormemory cell is in the high threshold voltage group, then Vs−Δ might beapplied to the bit line. Memory cells having Vs−Δ applied to them mayexperience an apparent upward shift of their threshold voltage by Δduring read. Therefore, Δ may be chosen based on the expected shift dueto cross-coupling due to the neighbor memory cell having a high Vt. Inone embodiment, Vd and Vd+Δ are applied to the bit lines.

In step 2008, Vcgry is applied to the selected word line and Vread isapplied to the unselected word lines. The voltage Vcgry may be selectedto create a Vgs for selected memory cells that is suitable to test for atarget Vt. For example, Vgs may be Vcgrv−Vs for memory cells having aneighbor with a low Vt. In this case, Vcgry−Vs might equal a thresholdvoltage that is being tested for. The Vgs for memory cells having aneighbor with a high threshold voltage may be Vcgry−(Vs−Δ). This Vgs mayalso test whether the threshold voltage of the selected memory cell isapproximately the target Vt, given the compensation for thecross-coupling effect.

In step 2010, the bit lines are sensed to determine whether the selectedmemory cells have a threshold voltage that is above/below the target Vt.The process may be repeated for other target threshold voltages.However, the initial step of determining the states of the neighborsneed not be repeated.

In one embodiment, cross-coupling compensation is performed duringprogramming. For example, WLs may be programmed one after another suchthat WLn+1 is programmed after WLn. In one embodiment, prior toprogramming a block of memory cells they are erased. Therefore, duringprogramming of target memory cells on WLn, the memory cells in WLn+1 areerased. Consequently, when WLn+1 is programmed, the read thresholdvoltage of cells in WLn may be shifted up as a function of the thresholdvoltage of the neighboring cells in WLn+1. In order to compensate forthis voltage shift, the verify level used when programming a targetmemory cell on WLn can be adjusted based on the intended state of theneighboring cells in WLn+1 (which is indicative to their thresholdvoltage after they will be programmed). For example, if the neighboringmemory cell on WLn+1 is going to remain in the erase state, then a“nominal” verify level “V” is used when verifying the target memory cellon WLn during its programming. On the other hand, if the neighboringmemory cell on WLn+1 is going to be programmed to another states (e.g.,the A-, B-, or C-state), then a verify level of V−Vs may be applied tothe bit line during verify of the target memory cell. The voltage Vs isa value that should compensate for expected cross coupling inducedapparent threshold voltage shift due to the later programming of theneighbor memory cells. Therefore, the voltage that is applied to the bitline may be a function of the state to which the neighbor memory cellwill be programmed.

Reading Soft Bits

In one embodiment, soft bits are read by reading simultaneously atdifferent threshold voltages. Reading soft bits can be used to boost ECCerror correction capability. FIG. 21 depicts memory cell thresholdvoltage distributions and will be used to help discuss reading softbits. FIG. 21 shows eight threshold voltage distributions. Adjacentthreshold voltage distributions overlap each other. In order todetermine the programmed state of each cell, the voltage window may bedivided into eight voltage bands, which can be done using seven readingthresholds Vr1,Vr2, . . . Vr7. If higher reading resolution is desiredin order to boost up ECC error correction capability, then additionalreadings can be performed near the voltage bands edges. In the exampleshown in FIG. 21, additional reads are performed at locations+Δ and −Δfrom the first locations (Vr1,Vr2, . . . Vr7). Therefore, measurementsat 21 different threshold voltages are gathered. The additional readingthresholds may be used for determining whether a cell's thresholdvoltage is close to the voltage band edge or not. This information maybe used for assigning a reliability estimate to the read cell's bits,which can be used by a “soft” ECC decoder. Further details of using softbits are described in Published U.S. Patent Application 2008/0244338,titled “Soft Bit Data Transmission for Error Correction Control inNon-volatile Memory,” filed on Mar. 31, 2007, which is herebyincorporated herein by reference for all purposes.

In one embodiment, first seven sense operations are done at readthresholds Vr1,Vr2, . . . , Vr7. These first seven sense operations maybe performed sequentially with each sense operation reading at one ofthe voltages. Then, seven additional sense operations may be performedwith each individual sense operation sensing at two different thresholdvoltages. For example, one sense operation senses at Vr1+Δ and at Vr1−Δ,then another senses at Vr2+Δ and at Vr2−Δ, and so on. Various techniquesdisclosed herein may be used to simultaneously sense the two differentthreshold voltages. This reduces the number of sense operations that areused.

FIG. 22 is a flowchart of one embodiment of a process of reading softbits. In step 2202, “n” sense operations are performed at “n” referencesvoltages. For example, seven sense operations are performed at locations(Vr1,Vr2, . . . Vr7). These initial locations may fall within theoverlapping ranges of threshold voltages between each of the datastates. For example, V1 falls in the range where states 0 and 1 overlap.Step 2202 determines whether the threshold voltage of each non-volatilestorage element is greater or less than each of the “n” referencesvoltages. This is one implementation of determining a suitable voltageto apply to each bit line based on a previous reading of a thresholdvoltage for the memory cell on the bit line (see FIG. 13D).

In step 2204, an additional sense operation is performed for each of the“n” sense operations. Each of the additional sense operations includessimultaneously sensing a first group of non-volatile storage elements inthe subset that have a threshold voltage greater than the “nth”reference voltage at a first offset voltage from the “nth” referencevoltage while sensing a second group of non-volatile storage elements inthe subset having a threshold voltage less than the “nth” referencevoltage at a second offset voltage from the “nth” reference voltage. Forexample, memory cells that have a threshold voltage greater than Vr1 aresensed at Vr1+Δ while memory cells that have a threshold voltage greaterthan Vr1 are sensed at Vr1−Δ. This is repeated for the other sevenlocations (Vr2, Vr3, . . . Vr7). Note that each of these senseoperations in step 2204 simultaneously senses two different thresholdvoltages.

In one embodiment, sensing at the first offset voltage from the “nth”reference voltage includes applying a first voltage difference acrossNAND strings having non-volatile storage elements in the first group andsensing first conditions of non-volatile storage elements in the firstgroup to determine whether the threshold voltages of non-volatilestorage elements in the first group is above or below the first offsetfrom the “nth” reference voltage. Furthermore, sensing at the secondoffset voltage from the “nth” reference voltage includes applying asecond drain-to-source voltage difference across NAND strings havingnon-volatile storage elements in the second group and sensing secondconditions of non-volatile storage elements in the second group todetermine whether the threshold voltages of non-volatile storageelements in the second group is above or below the second offset fromthe “nth” reference voltage.

In one embodiment, sensing at the first offset voltage from the “nth”reference voltage includes applying a first gate-to-source voltagedifference to non-volatile storage elements in the first group andsensing first conditions of non-volatile storage elements in the firstgroup in response to the first gate-to-source voltage difference todetermine whether the threshold voltages of non-volatile storageelements in the first group is above or below the first offset from the“nth” reference voltage. Furthermore, sensing at the second offsetvoltage from the “nth” reference voltage includes applying a secondgate-to-source voltage difference to non-volatile storage elements inthe second group and sensing second conditions of non-volatile storageelements in the second group in response to the second gate-to-sourcevoltage difference to determine whether the threshold voltages ofnon-volatile storage elements in the second group is above or below thesecond offset from the “nth” reference voltage.

In one embodiment, sensing at the first offset voltage from the “nth”reference voltage includes applying a first drain-to-source voltagedifference to non-volatile storage elements in the first group andsensing first conditions of non-volatile storage elements in the firstgroup in response to the first drain-to-source voltage difference todetermine whether the threshold voltages of non-volatile storageelements in the first group is above or below the first offset from the“nth” reference voltage. Furthermore, sensing at the second offsetvoltage from the “nth” reference voltage includes applying a seconddrain-to-source voltage difference to non-volatile storage elements inthe second group and sensing second conditions of non-volatile storageelements in the second group in response to the second drain-to-sourcevoltage difference to determine whether the threshold voltages ofnon-volatile storage elements in the second group is above or below thesecond offset from the “nth” reference voltage.

Performing a Binary Search

In one embodiment, a binary search for the threshold voltages of memorycells is efficiently performed by sensing for more than one thresholdvoltage at a time. FIG. 23A is a flowchart of one embodiment of aprocess of performing a binary search. The search efficiently determinesthe threshold voltages of many memory cells along a selected word line.The search takes place over a window of threshold voltages “W,” whichcan be any interval. FIG. 23B depicts an example range “W,” which willbe referred to when discussing FIG. 23A. Referring to FIG. 21, W mightinclude all states 0-7. Alternatively, W might range from about Vr1 toVr3 or some other small range.

In step 2302, sensing is performed at the midpoint of the window, whichwill be referred to as “W/2.” This step may involve applying a readvoltage to the selected word line while applying the same conditions toall of the selected bit lines.

In step 2304, bit lines are biased at one of two different levels tosense at W/4 and 3W/4. If a memory cell had a threshold voltage belowW/2, then it is sensed at W/4. If a memory cell had a threshold voltageabove W/2, then it is sensed at 3W/4. Note that step 2304 applies avoltage to the bit line that was determined based on a previous readingof the threshold voltage of the memory cell. In one embodiment, thedifferent bit line bias conditions cause a different Vgs for differentmemory cells to allow testing for different threshold voltages. In oneembodiment, the different bit line bias conditions cause a different Vdsfor different memory cells to allow testing for different thresholdvoltages. In one embodiment, the different bit line bias conditionscause different voltages across different NAND strings to allow testingfor different threshold voltages.

In step 2306, sensing is performed at W/4 and 3W/4. In step 2306, bothlevels (W/4 and 3W/4) are tested for with the same operation. Forexample, a read voltage may be applied to the selected word line whiledifferent bias conditions are applied to the two groups of bit lines.That is, for one group of memory cells are determination is made whethertheir threshold voltage is above/below W/4 and for a second group ofmemory cells are determination is made whether their threshold voltageis above/below 3W/4. These determinations may be made at the same time.For example, the determinations may be made by sensing some condition ofthe bit lines after applying the read voltage to the selected word line.

In step 2308, bit lines are biased at one of four different levels tosense at W/8, 3W/8, 5W/8, and 7W/8. If a memory cell had a thresholdvoltage below W/4, then it is sensed at W/4. If a memory cell had athreshold voltage between W/4 and W/2, then it is sensed at 3W/8. If amemory cell had a threshold voltage between W/2 and 3W/4, then it issensed at 5W/8. If a memory cell had a threshold voltage above 3W/4,then it is sensed at 7W/8. Therefore, a binary search is performed forthe threshold voltage of each memory cell. Note that step 2308 applies avoltage to the bit line that was determined based on a previous readingof the threshold voltage of the memory cell.

In one embodiment, the different bit line bias conditions cause adifferent Vgs for different memory cells to allow testing for differentthreshold voltages. In one embodiment, the different bit line biasconditions cause a different Vds for different memory cells to allowtesting for different threshold voltages. In one embodiment, thedifferent bit line bias conditions cause different voltages acrossdifferent NAND strings to allow testing for different thresholdvoltages. Note that it is not required that all four conditions betested for at the same time. Instead two conditions could be tested forat a time. For example, W/8 and 3W/8 could be tested for in oneoperation and 5W/8 and 7W/8 could be tested for in another operation.

In step 2310, sensing is performed at W/8, 3W/8, 5W/8, and 7W/8. Thatis, for one group of memory cells are determination is made whethertheir threshold voltage is above/below W/8, for a second group of memorycells are determination is made whether their threshold voltage isabove/below 3W/8, for a third group of memory cells are determination ismade whether their threshold voltage is above/below 5W/8, and for afourth group of memory cells are determination is made whether theirthreshold voltage is above/below 7W/8. These determinations may be madeat the same time. For example, the determinations may be made by sensingsome condition of the bit lines after applying a read voltage to theselected word line. The search can continue in a similar manner todetermine the threshold voltage at a finer level of granularity.

Example Sense Circuit

FIG. 24 depicts one embodiment of a sensing circuit which may be usedboth for biasing a bit line and sensing a condition of the bit line todetermine the threshold voltage of a selected memory cell on the bitline. The sensing circuit can perform “reverse sensing,” in which theselected memory cell current flows from the source line to the bit line.The sensing circuit is able to bias the bit line to several differentvoltages. A large number of sense circuits, e.g., 64K sense circuits ina plane, typically receive common control signals unless otherwiseindicated, and access one or more common power supplies. Regarding thepower supply, the transistor gate voltages in the sense circuit may beprovided by global circuits at the edge of large group of sensecircuits.

A separate copy of the sense circuit can be provided for each bit line,for every other bit line, or otherwise. However, by operating sensecircuits associated with different bit lines in a different manner,different bit line biases can be established. Therefore, different sensecircuit can test for different threshold voltages at the same time. Asanother example, different sense circuits can provide different amountsof cross-coupling compensation to different memory cells on a selectedword line. One or more control circuits can communicate commands to eachsense circuit to configure them, as well as to exchange data, such asread and write data.

The sense circuit can provide three different voltages to a bit line.For example, the sense circuit can pre-charge the bit line to one ofthree different voltages prior to sensing a condition of the bit line toread or verify the selected memory cell. Referring briefly to FIG. 9B,the sense amplifier could provide Vs, Vs−Δ, and a third voltage forunselected bit lines. For example, the sense amplifier could provide2.1, 1.5, and 2.5V to the bit line, respectively. It will be appreciatedthat modifications can be made to the sense amplifier to provideadditional voltages to bit lines. Therefore, with suitablemodifications, four different voltages can be provided to the bit linesas depicted in FIG. 18A.

The transistors in the sense circuits herein may include nMOSFETs(nMOSs) and pMOSFETs (pMOSs), for instance. Briefly, the sense circuitincludes bit line bias transistors, which help to establish the threedifferent voltages on the bit line 2468. The sense circuit also includessense bias transistors connected to a sense node (SEN). The sense biastransistors provide two different paths for sensing the bit line 2468.The sense circuit includes FLAG transistors that input a signal FLG andoutput INV. The sense circuit includes LATCH transistors that input asignal LAT and output INT. Details will be described below.

A bit line 2468 is connected to the sense circuit. The bit line 2468communicates with a BLS transistor 2470 and a BLC (bit line clamp)transistor 2472, which is coupled to a COM path. The BLS transistor 2472may be a high-voltage transistor which can isolate the sense circuit,which may include low voltage transistors, from high voltages of thememory array. During sensing, BLS transistor 2472 is conductive. In oneembodiment, the BLC transistor 2472 can clamp a voltage on the bit line2468 by control its gate voltage and supply of a sufficiently highvoltage such as Vdd on the drain of the transistor.

In one embodiment, a reference voltage is applied to the control gate ofa memory cell being read. If the reference voltage is greater than thethreshold voltage of the memory cell, then the memory cell will turn onand conduct current between its source and drain. If the referencevoltage is not greater than the threshold voltage of the memory cell,then the memory cell will not turn on and will not conduct currentbetween its source and drain. In many implementations, the on/off may bea continuous transition so that the memory cell will conduct differentcurrents in response to different control gate voltages. If the memorycell is on and conducting current, the conducted current will cause thevoltage on node SEN to change, effectively altering the voltage acrosscapacitor 2450. If the voltage on node SEN changes to a predeterminedlevel during a predetermined sensing period, then sense amplifierreports that the memory cell turned on in response to the control gatevoltage.

The sense circuit contains sense bias transistors 2456, 2458, 2460, and2462, which connect the bit line 2468 to the sense node SEN. Sensetransistors 2456 and 2460 are connected to the bit line throughtransistors 2472 and 2470. Sense transistors 2458 and 2462 are connectedto SEN. Sense bias transistors 2456 and 2458 provide one path betweentransistor 2472 and SEN. Sense bias transistors 2460 and 2462 provide asecond path between transistor 2472 and SEN. The sense node (SEN) isconnected to sense capacitor 2450, which is connected to a CLK signal.Signals INV, INT, XXP1, and XXP2 are applied to the gates of the sensetransistors to control sensing. Further details are discussed below inconnection with FIGS. 26A and 26B.

The sense node (SEN) is connected to the COM path (and hence to BLCtransistor 2472) via transistor 2454, which has signal XXO applied toits gate. The sense node (SEN) is also connected to transistor 2448,which has signal HHL applied to its gate. Transistor 2448 is connectedto Vsense, which is a voltage that SEN is set to just prior to sensing.For example, Vsense could be ground. Sense node SEN is connected to thenode BUS, which outputs a value indicative of whether the memory cellturned on. Specifically, SEN node is connected to the gate of transistor2446. Transistor 2446 is connected to BUS through transistors 2145 and2430. Therefore, signal STR may be applied to transistor 2415 and signalNCO may be applied to transistor 2430 to connect/disconnect SEN fromBUS.

The sense circuit includes bit line bias transistors 2402, 2404, 2406,2408, 2412, and 2412. Briefly, these transistors are used to bias thebit line to one of three different voltages. These transistors arecontrolled by signals INT, INV, FLA, FLB, FLC, and FLG. The signals INTand INV, are input from the output of FLAG and LATCH, respectively.Signals FLA, FLB, and FLC correspond to which of the three voltages thatthe bit line is to be biased. In one embodiment, control circuitrygenerates FLA, FLB, and FLC. Further details are discussed below.

As mentioned, FLAG inputs FLG and outputs INV. In one embodiment, theinitial value for INV is established by establishing a signal on BUS andturning on transistors 2430 and 2416 in order to establish FLG. Notethat transistor 2430 is controlled by NCO and transistor 2416 iscontrolled by FCO. Signals STF and FRB are applied to transistors 2420and 2422 to generate INV from FLG. Note the INV may be the opposite ofFLG.

As mentioned, LATCH inputs LAT and outputs INT. In one embodiment, theinitial value for INT is established by establishing a signal on BUS andturning on transistors 2430 and 2444 to establish LAT. Note thattransistor 2444 is controlled by LCO. Signals STL and PRS may be used togenerate INT from LAT.

FIGS. 25A, 25B, and 25C depict details of operation of the bit line biastransistors of FIG. 24 for a reverse sensing embodiment. FIGS. 25A, 25B,and 25C show voltages applied to gates of the bit line bias transistorsto establish the proper voltage on the bit lines prior to sensing thememory cell. Each of FIGS. 25A, 25B, and 25C depicts operation forbiasing the bit line to a different level. FIG. 25A shows operation forbiasing the bit line to the highest voltage. FIG. 25A depictstransistors 2410 and 2412 supplying a bias current to the bit line,which may be charged to Vdd (e.g., 2.5V). Note that Vbias may be a lowervoltage such as ground. FIG. 25B shows operation for biasing the bitline to a medium voltage. FIG. 25B depicts transistors 2404 and 2408sinking a bias current from the bit line, which may be charged to amedium voltage (e.g., 2.1V). Note that the charging current flows fromthe bit line to power source (Vbias). FIG. 25C shows operation forbiasing the bit line to a low voltage. FIG. 25C depicts transistors 2404and 2406 sinking a bias current from the bit line, which may be chargedto a low voltage (e.g., 1.5V). Note that the charging current flows fromthe bit line to power source (Vbias). As examples, the high voltagemight be used for biasing unselected bit lines, the medium voltage couldbe Vs (see FIG. 9B), and the low voltage could be Vs−Δ. Many otherpossibilities exist. Note that transistors 2402, 2404, 2406, and 2408are one embodiment of the current sink 1130 of FIGS. 11A and 11B. Notethat transistors 2410 and 2412 are one embodiment of the current source1230 of FIGS. 11A and 11B. Also note that transistors 2474, 2472 and2470 from FIG. 24 are not depicted between the sense bias transistorsand the bit line to simplify the drawing.

The table below provides example values for FLG, INV, LAT, and INT foroperation in FIGS. 25A, 25B and 25C. Recall from the discussion of FIG.24 that signals INV and INT are generated by FLAG and LATCH based on FLGand LAT. Therefore, these are signals that are generated internally inthe sense circuit. In one embodiment, a value of 0 for these signalscorresponds to 0V and a value of 1 corresponds to Vdd or 2.5V.

TABLE I Bit line Voltage FLG INV LAT INT High (e.g., Vdd or 2.5 V) 0 1 01 Medium (e.g., 2.1 V) 1 0 0 1 Low (e.g., 1.6 V) 1 0 1 0

The sense transistors also have signals FLA, FLB, and FLC applied togates of transistors 2410, 2408, and 2406, respectively. These signalsmay be common to many sense circuits. The timing for these signals aredepicted in the timing diagram in FIG. 27, which will be discussedbelow. In one embodiment, the values for these signals are as follows.The signal FLA may have a value of 0V when active. In one embodiment,FLB has a value of 2.1V−Vth of transistor 2408. The Vth of transistor2408 may be about 1.1V. Thus, FLB may be 2.1V−1.1V=1.0V. In oneembodiment, FLC has a value of 1.5V−Vth of transistor 2406. The Vth oftransistor 2406 may be about 1.1V. Thus, FLC may be 1.5V−1.1V=0.4V.

Operation of FIG. 25A will now be discussed beginning with the highvoltage path of transistors 2410 and 2412. Note that all of thetransistors in FIG. 25A are p-channel devices, and therefore turn onwhen a low voltage is applied to their gate. Also note that from FIG.24, the signal FLG may be applied to the gate of transistor 2412. Fromthe table, FLG=0, INV=Vdd, and INT=Vdd for high voltage operation. WhenFLG=0, transistor 2412 will turn. Signal FLA may be applied to the gateof transistor 2410. In one embodiment, FLA has a value of 0 when active.Therefore, signal FLA will turn on transistor 2410. Consequently, thebit line is connected to Vdd through transistors 2410 and 2412.

In order to prevent the medium and low voltage paths from conducting,Vdd may be applied to transistors 2402 and 2404. Recall that INT may beapplied to the gate of transistor 2402 and INV may be applied to thegate of transistor 2404. Because, INT and INV are both Vdd, Vdd may beapplied to the gates of transistors 2402 and 2404. Therefore,transistors 2402 and 2404 will not conduct.

Operation with respect to FIG. 25B will now be discussed. From thetable, FLG=Vdd, INV=0, and INT=Vdd for the medium voltage case. SinceFLG may have a value of Vdd, transistor 2412 will be off. Therefore,conduction is prevented on the high voltage path.

Operation of the medium voltage path is as follows for FIG. 25B. Notethat the bit line may be initially charged to about 2.5V. In oneembodiment, the bit line is initially charged to 2.5V by using thehigh-voltage path. The sense bias transistors 2404 and 2408 may then beused to reduce the voltage on the bit line. Transistor 2404 will be onbecause INV causes 0V on the gate of transistor 2404. FLB, which may beabout 1V, is applied to transistor 2408. Therefore, transistor 2408 willconduct as long as the bit line voltage is above 2.1V (recall Vth oftransistor 2408 is about 1.1V). Consequently, transistors 2408 and 2404reduce the bit line voltage until it reaches about 2.1V.

Operation of the low voltage path for FIG. 25B is as follows. Transistor2402 has Vdd applied to its gate as a result of INT being high.Therefore, transistor 2402 will not conduct, which prevents the lowvoltage path from controlling the bit line voltage.

Operation with respect to FIG. 25C will now be discussed. From thetable, FLG=Vdd, INV=0, and INT=0 for the low voltage case. Since FLG mayhave a value of Vdd, transistor 2412 will be off. Therefore, conductionis prevented along the high voltage path.

Operation of the low voltage path is as follows for FIG. 25C. Aspreviously mentioned, the bit line may be initially charged to about2.5V. For operation in FIG. 25C, transistor 2402 is on because INTcauses 0V to the gate of transistor 2402. Signal FLC, which may be about0.4V, is applied to the gate of transistor 2406. Therefore, transistor2406 will conduct as long as the bit line voltage is above 1.5V (recallthat the Vth of transistor 2406 is about 1.1V). Consequently,transistors 2402 and 2406 reduce the bit line voltage until it reachesabout 1.5V.

Operation of the medium voltage path is as follows for FIG. 25C.Transistor 2408 will have 1.0V applied to its gate as a result of signalFLB. Therefore, transistor 2408 can conduct a current if the bit line isabove 2.1V. However, after the bit line drops below 2.1V, transistor2408 turns off. Therefore, transistor 2408 may assist some indischarging the bit line, but will not impact the ability of transistor2406 to control the bit line voltage. Also note that transistor 2404 mayconduct because INV, which is applied to its gate is 0V.

FIGS. 26A and 26B depict voltages applied to sense bias transistors ofFIG. 24 during sensing. FIG. 26A depicts operation for the low voltagecase. For example, after the bit line bias transistors have been used toestablish 1.5V on the bit line, the sense bias transistors are used tosense some condition of the bit line. FIG. 26B depicts operation for themedium voltage case. For example, after the bit line bias transistorshave been used to establish 2.1V on the bit line, the sense biastransistors are used to sense some condition of the bit line. Thecircuits of FIGS. 26A and 26B are one implementation of the currentsense 1132 of FIG. 11B.

Referring briefly to FIG. 24, the signal INV is applied to transistor2456 and the signal INT is applied to transistor 2460. The signal XXP1is applied to transistor 2458, and signal XXP2 is applied to transistor2462. Signals XXP1 and XXP2 may be signals that are common to many sensecircuits. In one embodiment, XXP1 is about FLB+200 mV or about 1.2V. Inone embodiment, XXP2 is about FLC+200 mV or about 0.6V. Example timingof those signals is depicted in the timing diagram of FIG. 27, whichwill be discussed below. Values for INV and INT are depicted in table Iabove.

Operation of the circuit of FIG. 26A is as follows. The bit line wasbiased to about 1.5V by the bit line bias transistors for this case.Voltages to the gates of the transistors are based on values for INV,INT, XXP1 and XXP2. In general, transistors 2460 and 2462 will conductto connect the bit line to SEN. Transistor 2458 should be off becauseits gate is only about 0.3V below the bit line voltage. Therefore, itsgate is not low enough to turn on transistor 2458. Note that XX0 at thegate of transistor 2454 is low such that the COM path is not connectedto SEN.

Operation of the circuit of FIG. 26B is as follows. The bit line wasbiased to about 2.1V by the bit line bias transistors for this case.Voltages to the gates of the transistors are based on values for INV,INT, XXP1 and XXP2. Voltages are similar to the case of FIG. 26A, exceptnow Vdd is applied to the gate of transistor 2460. In general,transistors 2456 and 2458 will conduct to connect the bit line to SEN.Transistor 2460 should be off because its gate is at Vdd. Note that XX0at the gate of transistor 2454 is low such that the COM path is notconnected to SEN.

Note that for the high-voltage case sensing may not be desired. Forexample, unselected bit lines might be charged to the high voltage. Aspreviously mentioned, both INV and INT may be high (e.g., Vdd) for thehigh voltage case. Since INV is at the gate of transistor 2456 and INTis at the gate of transistor 2460, both sense bias paths will be shutoff. Therefore, SEN is not affected by the bit line.

FIG. 27 depicts a timing diagram associated with signals in the sensingcircuit of FIG. 24. Time points t0414 are depicted. The time lines arenot to scale. Prior to t0, SEN may have a high value that is a result ofa previous sense operation. The SEN node will be reset just prior to thesense operation (at t6). Therefore, the initial value of SEN is notcritical. Initially, BLC and BLY may be low such that the bit line isnot connected to the sense circuit. Initially, FLA, FLB, and FLC may behigh such that the high-, medium-, and low-voltage paths (see FIGS.25A-25C) are off. At time t0, the voltage Vsource is applied to thesource line.

Bit Line Pre-Charge Phase

At time t1, the signals BLC and BLY are raised to high, which turn ontransistors 2472 and 2474 respectively. Therefore, the bit line 2466 isconnected to the bit line bias transistors, as well as other elements inthe sense circuit. Also at time t1, the signals FLA, FLB, and FLC golow. This allows the high-, medium-, and low-voltage paths to be on/offdepending on the values for FLG, INV, and INT. Initially, the value forFLG may be low, which turns on transistor 2412. Therefore, thehigh-voltage path may be conductive at this time. However, at this timeINV and INT may be high such that the medium- and low-voltage paths willbe off. This allows the bit line to pre-charge through the high-voltagepath to Vdd (e.g., 2.5V).

Next, initial values for FLG, LAT, INT and INV are established. Table Idepicts example values for that may be used to help establish the high-,medium-, and low-voltage on the bit line. At time t2, the signal FCObriefly goes high until time t3. The signal NCO may also go high at thistime. This allows a signal from the BUS to set FLG. At time t4, thesignal LCO briefly goes high until time t5. The signal NCO may also behigh at this time. This allows a signal from the BUS to set LAT. Aspreviously discussed, INV and INT are generated by FLAG and LATCH basedon FLG and LAT, respectively.

The values for FLG, INV, and INT are applied to gates of the bit linebias transistors, as depicted in FIG. 24. Operation of the bit line biastransistors has already been described in connection with FIGS. 25A-25C.The bit line voltage is allowed to stabilize until t6.

Sensing Phase

Signal HLL goes high from time t6 until time t7. This briefly turns onthe HLL transistor 2448 to establish an initial voltage at SEN at thebeginning of a sense operation. In one embodiment, SEN is pulled toground.

Signals XXP1 and XXP2 begin to go low at time t8. By time t9, signalsXXP1 and XXP2 are low. Recall that signals XXP1 and XXP2 are input tosense bias transistors 2458 and 2462, respectively. By this time, INVand INT will be established for the desired bit line voltage condition.Therefore, operation of the sense bias transistors is as described inconnection with FIGS. 26A and 26B.

At time t9, the sense node (SEN) begins to react based on a condition ofthe bit line. As previously discussed the bit line will conduct acurrent whose magnitude depends, at least in part, on the thresholdvoltage of the selected memory cell. The conduction current (if any)travels in the direction depicted in FIGS. 9B and 11B. Therefore, theconduction current may charge the capacitor 2450 at the sense node SEN.As a result, the voltage SEN may increase as the capacitor 2450 chargesas a result of the conduction current in the bit line.

The SEN node is depicted as rising at three different rates. The rate ofincrease may depend in part on the difference in voltage between the bitline and SEN. Therefore, in general, the rate of increase may be fasterfor the medium-voltage bit line case than for the low-voltage bit linecase. The rate of increase may also depend in part on the magnitude ofthe memory cell conduction current. The lines labeled A and B correspondto the cases where the memory cell conduction current is high. Line Acorresponds to the medium-voltage bit line case. Line B corresponds tothe low-voltage bit line case. Therefore, line A goes up at a higherrate than line B. The line labeled C corresponds to the cases where thememory cell conduction current is low. Only one line (C) is depicted forthe low conduction current cases; however, there may be some differencein the rate of increase depending on the bit line voltage.

At time t10, signals XXP1 and XXP2 begin to go high again to stop thecap 2450 from charging up further. By time t11, signals XXP1 and XXP2are again high. At time t11, CLK and FCO go high. Note when CLK goeshigh, that the voltage at SEN may be affected by CLK (see FIG. 24). TheSEN voltage may not be high enough to trigger the set of the latches,thus CLK step up will boost the level of SEN voltage for latching.

Latch Phase

The strobe signal STR is brought high between time t12 and t13. BecauseFCO is on, the voltage from SEN is inversely passed to FLG through STRand FCO. The value of FLG may be stored in FLAG latch. At time t11 FCOis brought back low. Note that LCO could also be brought high to passthe signal from SEN to LAT. This value may then be stored in LATCH.

Read Out Phase

Finally, in a read out phase at t14, the control signal NCO at thetransfer gate 2430 allows the signal SEN to be read out to the readoutBUS. The cell passed either A or B state will be latched into FLAG. Thedata may be sorted out when the data is shifted out through NCO to alogic processor. The data may be selected based on the program datastored in the data latches outside of the sense amplifier. For example,if it is A state program data, the FLG latch data high is an indicationthat the cell threshold voltage passed state A. If the data is B stateprogram data, the FLG latch data high is a indication that the cellpassed state B.

One embodiment is a method for operating non-volatile storage thatincludes a plurality of NAND strings of non-volatile storage elements.The method includes causing a first voltage difference across a firstNAND string of the plurality of NAND strings that includes a firstnon-volatile storage element and causing a second voltage differenceacross a second NAND string of the plurality of NAND strings thatinclude a second non-volatile storage element. The second voltagedifference is different from the first voltage difference. The secondvoltage difference is caused at the same time that the first voltagedifference is caused. A read voltage is applied to a word line that isassociated with the first non-volatile storage element and the secondnon-volatile storage element while causing the first and second voltagedifferences. A first condition of the first non-volatile storage elementis sensed in response to the first voltage difference and read voltageto determine whether the threshold voltage of the first non-volatilestorage element is above or below a first reference voltage. A secondcondition of the second non-volatile storage element is sensed inresponse to the second voltage difference and read voltage to determinewhether the threshold voltage of the second non-volatile storage elementis above or below a second reference voltage. In one embodiment, thefirst reference voltage is different from the second reference voltage.In one embodiment, the first reference voltage is the same as the secondreference voltage.

With some slight modifications, the sense circuit of FIG. 24 may be usedfor forward sensing. FIG. 28A depicts one embodiment of a circuit thatcan be used in place of the bit line bias transistors in FIG. 24. Thecircuit of FIG. 28A is different in that p-channel transistors 2406 and2408 are replaced with n-channel transistors 2806 and 2808. Also, Vdd(e.g., 2.5V) is supplied to all transistors 2402, 2404, and 2412, asopposed to supplying Vbias to transistors 2402 and 2404. Operation issimilar to the reverse sensing embodiment described with respect toFIGS. 25A-25C. However, the paths that provide the low- and medium bitline voltages are switched. As with the reverse sensing embodiment, FLGis applied to the gate of transistor 2412, FLA is applied to the gate oftransistor 2410, FLB is applied to the gate of transistor 2808, and FLAis applied to the gate of transistor 2806. However, the signals FLA, FLBand FLC are different than in the reverse sensing embodiment.

FIG. 29 depicts a timing diagram of one embodiment of signals forforwarding sensing, including signals FLA, FLB, and FLC. In oneembodiment, the values for FLA, FLB, and FLC are as follows. The signalFLA may have a value of 0V when active. In one embodiment, FLB has avalue of 1.5V+Vtn of transistor 2808. The Vtn of transistor 2408 may beabout 0.6V. Thus, FLB may be 1.5V+0.6V=2.1V. In one embodiment, FLC hasa value of 2.1V+Vtn of transistor 2806. The Vtn of transistor 2406 maybe about 0.6V. Thus, FLC may be 2.1V+0.6V=2.7V.

TABLE II Bit line Voltage FLG INV LAT INT High (e.g., Vdd or 2.5 V) 0 10 1 Medium (e.g., 2.1 V) 1 0 1 0 Low (e.g., 1.5 V) 1 0 0 1

Referring again to FIG. 28A, the signals from Table II may be used inthe forward sensing embodiment. These signals are similar to those inTable I, but with the medium- and low-voltage cases switched. Thus, tobias the bit line to the highest voltage, FLG may be 0V, while INT andINV may both be Vdd. When signal FLA is 0V, the bit line may be chargedto Vdd (e.g., 2.5V) due to transistors 2410 and 2412 being on. BecauseINT and INV are both Vdd conduction along the other paths is prevented.

From Table II, FLG=Vdd, INV=0, and INT=Vdd for the low voltage case.Since FLG is Vdd, transistor 2412 will be off. Therefore, conduction isprevented on the high voltage path. Transistor 2404 will be on becauseINV causes 0V on the gate of transistor 2404. FLB, which may be about2.1V, is applied to transistor 2808. Therefore, transistor 2808 willconduct until the bit line voltage reaches 1.5V. Consequently,transistors 2808 and 2404 increase the bit line voltage until it reachesabout 1.5V. Since, INT=Vdd for the low-voltage case, transistor 2402will be off.

From Table II, FLG=Vdd, INV=0, and INT=0 for the medium-voltage case.Since FLG may have a value of Vdd, transistor 2412 will be off.Transistor 2402 is on because INT causes 0V to the gate of transistor2402. Signal FLC, which may be about 2.7V, is applied to the gate oftransistor 2806. Therefore, transistor 2806 will conduct until the bitline voltage reaches 2.1V. Consequently, transistors 2402 and 2806increase the bit line voltage until it reaches about 2.1V. Note thattransistor 2404 may conduct because INV, which is applied to its gate is0V. Transistor 2408 will have 2.1V applied to its gate as a result ofsignal FLB. Therefore, transistor 2808 can conduct a current until thebit line reaches 1.5V. However, after the bit line reaches 1.5V,transistor 2808 turns off. Therefore, transistor 2808 may assist some incharging the bit line, but will not impact the ability of transistor2806 to control the bit line voltage.

FIG. 28B depicts one embodiment of a circuit of sense transistors forforward sensing. Sensing is performed through transistor 2454 forforward sensing. For forward sensing, current flows from the sense node(SEN) to the bit line in the direction depicted by the arrow. Thus, thesense bias transistors 2456, 2458, 2460, and 2462 of FIG. 24 are notrequired for forward sensing.

FIG. 29 depicts a timing diagram of signals for one embodiment offorward sensing. The signals refer to those in FIG. 24, as modified byreplacing the bit line bias transistors with the circuit of FIG. 28A.Prior to t0, SEN may have a high value that may be a result of resettingthe SEN node to that value. Initially, BLC and BLY may be low such thatthe bit line is not connected to the sense circuit. Initially, FLA maybe high and FLB and FLC may be low such that the high-, medium-, andlow-voltage paths (see FIG. 28A) are off. At time t0, the voltageVsource is applied to the source line In one embodiment, Vsource equalsVdd.

Bit Line Pre-Charge Phase

At time t1, the signals BLC and BLY are raised to high, which turn ontransistors 2472 and 2474 respectively. Therefore, the bit line 2466 isconnected to the bit line bias transistors, as well as other elements inthe sense circuit. Also at time t1, the signal FLA begins to transitionto low (e.g., 0V), and FLB and FLC begin to transition to high (e.g.,2.1V and 2.7V respectively). This allows the high-, medium-, andlow-voltage paths to be on/off depending on the values for FLG, INV, andINT.

Next, initial values for FLG, LAT, INT and INV are established. Table IIdepicts example values for that may be used to help establish the high-,medium-, and low-voltage on the bit line for forward sensing. At timet2, the signal FCO briefly goes high until time t3. This allows a signalfrom the BUS to set FLG. The values for FLG, INV, and INT are applied togates of the bit line bias transistors, as depicted in FIG. 28A.Operation of the bit line bias transistors has already been described inconnection with FIG. 28A.

First Sensing Phase

Signal HLL goes high from time t4 until time t5. This briefly turns onthe HLL transistor 2448 to establish an initial voltage at SEN at thebeginning of a sense operation. In one embodiment, Vsense is a highvoltage to raise SEN to a high voltage.

Signal XX0 begins to go high at time t5 and returns to low by time t6.In one embodiment, XX0 is raised to FLB+200 mV. When XX0 is high, thesense node (SEN) is connected to the bit line. At time t5, the sensenode (SEN) begins to react based on a condition of the bit line. Aspreviously discussed, the bit line will conduct a current whosemagnitude depends, at least in part, on the threshold voltage of theselected memory cell. The conduction current (if any) travels in theforward direction depicted in FIGS. 9C and 12B. Therefore, theconduction current may discharge the capacitor 2450 at the sense nodeSEN (see FIG. 28B). As a result, the voltage SEN may decrease as thecapacitor 2450 discharges as a result of the conduction current in thebit line.

The SEN node is depicted as falling at three different rates. The rateof decrease may depend in part on the difference in voltage between thebit line and SEN. Therefore, in general, the rate of decrease may befaster for the medium-voltage bit line case than for the low-voltage bitline case. The rate of decrease may also depend in part on the magnitudeof the memory cell conduction current. The lines labeled A and Bcorrespond to the cases where the memory cell conduction current ishigh. Line A corresponds to the low-voltage bit line case. Line Bcorresponds to the medium-voltage bit line case. Therefore, line A goesdown at a faster rate than line B. The line labeled C corresponds to thecases where the memory cell conduction current is low. Only one line (C)is depicted for the low conduction current cases; however, there may besome difference in the rate of increase depending on the bit linevoltage.

Latch Phase

By time t6, signal XX0 is down to low again to stop the capacitor 2450from discharging further. At time t7, CLK and FCO go high. The CLKsignal couples up the SEN node such that a SEN voltage of certain levelwill turn on transistor 2446 in order to differentiate non-conductingmemory cells from conducting memory cells.

The strobe signal STR is brought high between time t8 and t9. BecauseFCO is on, the voltage from SEN is inversely passed to FLG through STRand FCO. The value of FLG may be stored in FLAG latch. At time t9 FCOand CLK are brought back low.

Second Sensing Phase

Signal HLL goes high again from time t9 until time t10. This brieflyturns on the HLL transistor 2448 to re-establish an initial voltage atSEN at the beginning of a sense operation. In one embodiment, Vsense isa high voltage to raise SEN to a high voltage.

Signal XX0 begins to go high at time t10 and returns to low by time t11.In one embodiment, XX0 is raised to FLC+200 mV. At time t10, the sensenode (SEN) begins to react based on a condition of the bit line. By timet11, signal XX0 is down to low again to stop the capacitor 2450 fromdischarging further.

Latch Phase

At time t12, CLK and LCO go high. The strobe signal STR is brought highbetween time t12 and t13. Because LCO is on, the voltage from SEN isinversely passed to LATCH through STR and LCO. The value of LAT may bestored in LATCH latch. At time t13 LCO and CLK are brought back low.

Read Out Phase

Similar to the timing diagram of FIG. 27, there may be a readout phasein which NCO is brought high. The control signal NCO at the transfergate 2430 allows the signal SEN to be read out to the readout BUS.

One embodiment is a method for operating non-volatile storage thatincludes the following. A first voltage is established on a commonsource line that is electrically connected to a plurality of NANDstrings of non-volatile storage elements that are programmed to at leastfour states. A second voltage, which is lower than the first voltage, isestablished on a first bit line that is associated with a first NANDstring of the plurality of NAND strings, the first NAND string includesa first non-volatile storage element. A third voltage, which is lowerthan the first voltage, is established on a second bit line that isassociated with a second NAND string of the plurality of NAND strings.The second voltage is different from the first voltage. The second NANDstring includes a second non-volatile storage element. A fourth voltageis established on a word line that is associated with the plurality ofNAND strings. The fourth voltage is established while the second voltageand the third voltage are established on the first and second bit lines.A first condition of the first non-volatile storage element is sensed inresponse to establishing the fourth voltage on the word line todetermine whether the threshold voltage of the first non-volatilestorage element is above or below a first reference voltage. A secondcondition of the second non-volatile storage element is sensed inresponse to establishing the fourth voltage on the word line todetermine whether the threshold voltage of the second non-volatilestorage element is above or below a second reference voltage. In oneembodiment, the first reference voltage is different from the secondreference voltage. In one embodiment, the first reference voltage is thesame as the second reference voltage.

One embodiment is method for operating non-volatile storage thatincludes a plurality of NAND strings of non-volatile storage elements.The method includes the following. A determination of whether thethreshold voltage of a first non-volatile storage element that isassociated with a first NAND string of the plurality of NAND strings isabove or below a first reference voltage within a voltage range is made.A determination of whether the threshold voltage of a secondnon-volatile storage element that is associated with a second NANDstring of the plurality of NAND strings is above or below the firstreference voltage is made. A determination of a first voltage differenceto apply across the first NAND string based on the threshold voltage ofthe first non-volatile storage element is made. A determination ofsecond voltage difference to apply across the second NAND string basedon the threshold voltage of the second non-volatile storage element ismade. The first voltage difference is different than the second voltagedifference. The first voltage difference is caused across the first NANDstring while causing the second voltage difference across the secondNAND string. A read voltage is applied to a word line that is associatedwith the first non-volatile storage element and the second non-volatilestorage element while causing the first and second voltage differences.A first condition of the first non-volatile storage element is sensed inresponse to the first voltage difference and read voltage to determinewhether the threshold voltage of the first non-volatile storage elementis above or below a second reference voltage within the voltage range. Asecond condition of the second non-volatile storage element is sensed inresponse to the second voltage difference and read voltage to determinewhether the threshold voltage of the second non-volatile storage elementis above or below a third reference voltage within the voltage range,the third reference voltage is different from the second referencevoltage.

One embodiment is a method for operating non-volatile storage thatincludes a plurality of NAND strings of non-volatile storage elementsthat are programmed to “n+1” data states. Each data state correspondingto a range of threshold voltages; adjacent data states have overlappingranges of threshold voltages. The non-volatile storage elements eachhave a gate and a source. The method includes performing “n” senseoperations at “n” references voltages that fall within the overlappingranges of threshold voltages between each of the data states todetermine whether the threshold voltage of each non-volatile storageelement in a subset of the non-volatile storage elements is greater orless than each of the “n” references voltages. An additional senseoperation is performed for each of the “n” sense operations. Each of theadditional sense operations includes simultaneously sensing a firstgroup of non-volatile storage elements in the subset having a thresholdvoltage greater than the “nth” reference voltage at a first offsetvoltage from the “nth” reference voltage while sensing a second group ofnon-volatile storage elements in the subset having a threshold voltageless than the “nth” reference voltage at a second offset voltage fromthe “nth” reference voltage. The first and second offsets are different.Sensing is performed at the first offset voltage from the “nth”reference voltage, which includes applying a first gate-to-sourcevoltage difference to non-volatile storage elements in the first groupand sensing first conditions of non-volatile storage elements in thefirst group in response to the first gate-to-source voltage differenceto determine whether the threshold voltages of non-volatile storageelements in the first group is above or below the first offset from the“nth” reference voltage. Sensing is performed at the second offsetvoltage from the “nth” reference voltage, which includes applying asecond gate-to-source voltage difference to non-volatile storageelements in the second group and sensing second conditions ofnon-volatile storage elements in the second group in response to thesecond gate-to-source voltage difference to determine whether thethreshold voltages of non-volatile storage elements in the second groupis above or below the second offset from the “nth” reference voltage.

One embodiment includes non-volatile storage device that includes aplurality of NAND strings of non-volatile storage elements. The NANDstrings include a first NAND string having a first non-volatile storageelement and a second NAND string having a second non-volatile storageelement. The non-volatile storage device includes a plurality of wordlines associated with the plurality of NAND strings and one or moremanaging circuits in communication with the non-volatile storageelements. The one or more managing circuits cause a first voltagedifference across the first NAND string. The one or more managingcircuits cause a second voltage difference across the second NANDstring. The second voltage difference is different from the firstvoltage difference. The second voltage difference is caused at the sametime that the first voltage difference is caused. The one or moremanaging circuits apply a read voltage to a first of the word lineswhile causing the first and second voltage differences. The one or moremanaging circuits sense a first condition of the first non-volatilestorage element in response to the first voltage difference and readvoltage to determine whether the threshold voltage of the firstnon-volatile storage element is above or below a first referencevoltage. The one or more managing circuits sense a second condition ofthe second non-volatile storage element in response to the secondvoltage difference and read voltage to determine whether the thresholdvoltage of the second non-volatile storage element is above or below asecond reference voltage. In one embodiment, the first reference voltageis different from the second reference voltage. In one embodiment, thefirst reference voltage is the same as the second reference voltage.

One embodiment is a non-volatile storage device including a plurality ofNAND strings of non-volatile storage elements. The NAND strings includea first NAND string having a first non-volatile storage element and asecond NAND string having a second non-volatile storage element. Thenon-volatile storage device further includes a plurality of bit lines, afirst of the bit lines is associated with the first NAND string, asecond of the bit lines is associated with a second of the NAND strings.The non-volatile storage device further includes a plurality of wordlines associated with the plurality of NAND strings and a common sourceline that is electrically connected to the plurality of NAND strings.The non-volatile storage device further includes one or more managingcircuits in communication with the non-volatile storage elements. Theone or more managing circuits establish a first voltage on the commonsource line. The one or more managing circuits establish a secondvoltage on the first bit line, the one or more managing circuitsestablish a third voltage on the second bit line. The second voltage isdifferent from the third condition. The second voltage is less than thefirst voltage. The third voltage is less than the first voltage. The oneor more managing circuits establish a fourth voltage on a selected wordline. The fourth voltage is established while the second voltage and thethird voltage are established on the first and second bit lines. The oneor more managing circuits sense a first condition of the firstnon-volatile storage element in response to establishing the fourthvoltage on the selected word line to determine whether the thresholdvoltage of the first non-volatile storage element is above or below afirst reference voltage. The one or more managing circuits sense asecond condition of the second non-volatile storage element in responseto establishing the fourth voltage on the selected word line todetermine whether the threshold voltage of the second non-volatilestorage element is above or below a second reference voltage. In oneembodiment, the first reference voltage is different from the secondreference voltage. In one embodiment, the first reference voltage is thesame as the second reference voltage.

One embodiment is a non-volatile storage device including a plurality ofNAND strings of non-volatile storage elements. The NAND strings includea first NAND string having a first non-volatile storage element and asecond NAND string having a second non-volatile storage element. Thenon-volatile storage device further includes a plurality of word linesassociated with the plurality of NAND strings. The non-volatile storagedevice further includes one or more managing circuits in communicationwith the non-volatile storage elements. The one or more managingcircuits determine whether the threshold voltage of the firstnon-volatile storage element is above or below a first reference voltagewithin a voltage range. The one or more managing circuits determinewhether the threshold voltage of the second non-volatile storage elementis above or below the first reference voltage. The one or more managingcircuits determine a first voltage difference to apply across the firstNAND string based on the threshold voltage of the first non-volatilestorage element. The one or more managing circuits determine a secondvoltage difference to apply across the second NAND string based on thethreshold voltage of the second non-volatile storage element, the firstvoltage difference is different than the second voltage difference, theone or more managing circuits cause the first voltage difference acrossthe first NAND string while causing the second voltage difference acrossthe second NAND string. The one or more managing circuits apply a readvoltage to a selected word line that is associated with the firstnon-volatile storage element and with the second non-volatile storageelement while causing the first and second voltage differences. The oneor more managing circuits sense a first condition of the firstnon-volatile storage element in response to the first voltage differenceand read voltage to determine whether the threshold voltage of the firstnon-volatile storage element is above or below a second referencevoltage within the voltage range, the one or more managing circuitssense a second condition of the second non-volatile storage element inresponse to the second voltage difference and read voltage to determinewhether the threshold voltage of the second non-volatile storage elementis above or below a third reference voltage within the voltage range,the third reference voltage is different from the second referencevoltage.

One embodiment includes a non-volatile storage device including aplurality of NAND strings of non-volatile storage elements. The NANDstrings include a first NAND string having a first non-volatile storageelement and a second NAND string having a second non-volatile storageelement, the non-volatile storage elements each having a gate and asource. The non-volatile storage device includes a plurality of wordlines associated with the plurality of NAND strings and one or moremanaging circuits in communication with the non-volatile storageelements. The one or more managing circuits program the non-volatilestorage elements to “n+1” data states, each data state corresponding toa range of threshold voltages, adjacent data states having overlappingranges of threshold voltages. The one or more managing circuits perform“n” sense operations at “n” references voltages that fall within theoverlapping ranges of threshold voltages between each of the data statesto determine whether the threshold voltage of each non-volatile storageelement in a subset of the non-volatile storage elements is greater orless than each of the “n” references voltages. The one or more managingcircuits perform an additional sense operation for each of the “n” senseoperations. Each of the additional sense operations includessimultaneously sensing a first group of non-volatile storage elements inthe subset having a threshold voltage greater than the “nth” referencevoltage at a first offset voltage from the “nth” reference voltage whilesensing a second group of non-volatile storage elements in the subsethaving a threshold voltage less than the “nth” reference voltage at asecond offset voltage from the “nth” reference voltage. The first andsecond offsets are different. The one or more managing circuits sense atthe first offset voltage from the “nth” reference voltage includesapplying a first gate-to-source voltage difference to non-volatilestorage elements in the first group and sensing first conditions ofnon-volatile storage elements in the first group in response to thefirst gate-to-source voltage difference to determine whether thethreshold voltages of non-volatile storage elements in the first groupis above or below the first offset from the “nth” reference voltage. Theone or more managing circuits sense at the second offset voltage fromthe “nth” reference voltage, which includes applying a secondgate-to-source voltage difference to non-volatile storage elements inthe second group and sensing second conditions of non-volatile storageelements in the second group in response to the second gate-to-sourcevoltage difference to determine whether the threshold voltages ofnon-volatile storage elements in the second group is above or below thesecond offset from the “nth” reference voltage.

The foregoing detailed description of the invention has been presentedfor purposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed. Manymodifications and variations are possible in light of the aboveteaching. The described embodiments were chosen in order to best explainthe principles of the invention and its practical application, tothereby enable others skilled in the art to best utilize the inventionin various embodiments and with various modifications as are suited tothe particular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto.

1. A method for operating non-volatile storage, comprising: establishinga first voltage on a common source line that is electrically connectedto a plurality of NAND strings of non-volatile storage elements that areprogrammed to at least four states; establishing a second voltage on afirst bit line that is associated with a first NAND string of theplurality of NAND strings, the first NAND string includes a firstnon-volatile storage element, the second voltage is less than the firstvoltage; establishing a third voltage on a second bit line that isassociated with a second NAND string of the plurality of NAND strings,the second condition is different from the first condition, the secondNAND string includes a second non-volatile storage element, the thirdvoltage is less than the first voltage; establishing a fourth voltage ona word line that is associated with the plurality of NAND strings, thefourth voltage is established while the second voltage and the thirdvoltage are established on the first and second bit lines; sensing afirst condition of the first non-volatile storage element in response toestablishing the second voltage on the word line to determine whetherthe threshold voltage of the first non-volatile storage element is aboveor below a first reference voltage; and sensing a second condition ofthe second non-volatile storage element in response to establishing thesecond voltage on the word line to determine whether the thresholdvoltage of the second non-volatile storage element is above or below asecond reference voltage.
 2. The method of claim 1 wherein the firstreference voltage is different from the second reference voltage.
 3. Themethod of claim 1 wherein the first reference voltage is the same as thesecond reference voltage.
 4. The method of claim 1, wherein establishingthe second voltage is based on a first state of a plurality of states towhich the first non-volatile storage element is being programmed andestablishing the third voltage is based on a second state of theplurality of states to which the second non-volatile storage element isbeing programmed.
 5. The method of claim 1, further comprising:establishing the second voltage on a third bit line that is associatedwith a third NAND string of the plurality of NAND strings, the thirdNAND string includes a third non-volatile storage element; establishingthe third voltage on a fourth bit line that is associated with a fourthNAND string of the plurality of NAND strings, the fourth NAND stringincludes a fourth non-volatile storage element; raising the voltage onthe word line to a fifth voltage after sensing the first and secondconditions; biasing the first bit line and the third bit line at thefirst voltage after sensing the third and fourth conditions; sensing athird condition of the third non-volatile storage element in response toraising the voltage on the word line to the fifth voltage to determinewhether the third non-volatile storage element is programmed to a thirddata state of the plurality of states; and sensing a fourth condition ofthe fourth non-volatile storage element in response to raising thevoltage on word line to the fifth voltage to determine whether thefourth non-volatile storage element is programmed to a fourth data stateof the plurality of states.
 6. A method for operating non-volatilestorage that includes a plurality of NAND strings of non-volatilestorage elements, the method comprising: determining whether thethreshold voltage of a first non-volatile storage element that isassociated with a first NAND string of the plurality of NAND strings isabove or below a first reference voltage within a voltage range;determining whether the threshold voltage of a second non-volatilestorage element that is associated with a second NAND string of theplurality of NAND strings is above or below the first reference voltage;determining a first voltage difference to apply across the first NANDstring based on the threshold voltage of the first non-volatile storageelement; determining a second voltage difference to apply across thesecond NAND string based on the threshold voltage of the secondnon-volatile storage element, the first voltage difference is differentthan the second voltage difference; causing the first voltage differenceacross the first NAND string while causing the second voltage differenceacross the second NAND string; applying a read voltage to a word linethat is associated with the first non-volatile storage element and thesecond non-volatile storage element while causing the first and secondvoltage differences; sensing a first condition of the first non-volatilestorage element in response to the first voltage difference and readvoltage to determine whether the threshold voltage of the firstnon-volatile storage element is above or below a second referencevoltage within the voltage range; and sensing a second condition of thesecond non-volatile storage element in response to the second voltagedifference and read voltage to determine whether the threshold voltageof the second non-volatile storage element is above or below a thirdreference voltage within the voltage range, the third reference voltageis different from the second reference voltage.
 7. The method of claim 6wherein the voltage range has an upper voltage and a lower voltage, thefirst reference voltage is midway between the lower voltage and theupper voltage, the second reference voltage is half-way between thefirst reference voltage and the upper voltage, the third referencevoltage is half-way between the first reference voltage and the lowervoltage.
 8. The method of claim 6 further comprising determining a thirdvoltage difference to apply across the first NAND string based onwhether the threshold voltage of the first non-volatile storage elementis above or below the second reference voltage; determining a fourthvoltage difference to apply across the second NAND string based onwhether the threshold voltage of the second non-volatile storage elementis above or below the third reference voltage; causing the third voltagedifference across the first NAND string while causing the fourth voltagedifference across the second NAND string; applying the read voltage tothe word line while causing the third and fourth voltage differences;sensing a third condition of the first non-volatile storage element inresponse to the third voltage difference and read voltage to determinewhether the threshold voltage of the first non-volatile storage elementis above or below a fourth reference voltage within the voltage range;and sensing a fourth condition of the second non-volatile storageelement in response to the fourth voltage difference and read voltage todetermine whether the threshold voltage of the second non-volatilestorage element is above or below a fifth reference voltage within thevoltage range, the fifth reference voltage is different from the fourthreference voltage.
 9. The method of claim 8 wherein the voltage rangehas an upper voltage and a lower voltage, the first reference voltage ismidway between the lower voltage and the upper voltage, the secondreference voltage is half-way between the first reference voltage andthe upper voltage, the third reference voltage is half-way between thefirst reference voltage and the lower voltage, the fourth referencevoltage is either midway between the second voltage and the uppervoltage or midway between the first voltage and the second voltage, thefifth reference voltage is either midway between the third voltage andthe lower voltage or midway between the first voltage and the thirdvoltage.
 10. A method for operating non-volatile storage that includes aplurality of NAND strings of non-volatile storage elements that areprogrammed to “n+1” data states, each data state corresponding to arange of threshold voltages, adjacent data states having overlappingranges of threshold voltages, the non-volatile storage elements eachhaving a gate and a source, the method comprising: performing “n” senseoperations at “n” references voltages that fall within the overlappingranges of threshold voltages between each of the data states todetermine whether the threshold voltage of each non-volatile storageelement in a subset of the non-volatile storage elements is greater orless than each of the “n” references voltages; performing an additionalsense operation for each of the “n” sense operations, each of theadditional sense operations includes simultaneously sensing a firstgroup of non-volatile storage elements in the subset having a thresholdvoltage greater than the “nth” reference voltage at a first offsetvoltage from the “nth” reference voltage while sensing a second group ofnon-volatile storage elements in the subset having a threshold voltageless than the “nth” reference voltage at a second offset voltage fromthe “nth” reference voltage, the first and second offsets are different;sensing at the first offset voltage from the “nth” reference voltageincludes applying a first gate-to-source voltage difference tonon-volatile storage elements in the first group and sensing firstconditions of non-volatile storage elements in the first group inresponse to the first gate-to-source voltage difference to determinewhether the threshold voltages of non-volatile storage elements in thefirst group is above or below the first offset from the “nth” referencevoltage; sensing at the second offset voltage from the “nth” referencevoltage includes applying a second gate-to-source voltage difference tonon-volatile storage elements in the second group and sensing secondconditions of non-volatile storage elements in the second group inresponse to the second gate-to-source voltage difference to determinewhether the threshold voltages of non-volatile storage elements in thesecond group is above or below the second offset from the “nth”reference voltage.
 11. A non-volatile storage device comprising: aplurality of NAND strings of non-volatile storage elements, the NANDstrings include a first NAND string having a first non-volatile storageelement and a second NAND string having a second non-volatile storageelement; a plurality of bit lines, a first of the bit lines isassociated with the first NAND string, a second of the bit lines isassociated with a second of the NAND strings; a plurality of word linesassociated with the plurality of NAND strings; a common source line thatis electrically connected to the plurality of NAND strings; and one ormore managing circuits in communication with the non-volatile storageelements, the one or more managing circuits establish a first voltage onthe common source line, the one or more managing circuits establish asecond voltage on the first bit line, the second voltage is less thanthe first voltage, the one or more managing circuits establish a thirdvoltage on the second bit line, the third voltage is less than the firstvoltage, the second voltage is different from the third voltage, the oneor more managing circuits establish a fourth voltage on a selected wordline, the fourth voltage is established while the second voltage and thethird voltage are established on the first and second bit lines, the oneor more managing circuits sense a first condition of the firstnon-volatile storage element in response to establishing the fourthvoltage on the selected word line to determine whether the thresholdvoltage of the first non-volatile storage element is above or below afirst reference voltage, the one or more managing circuits sense asecond condition of the second non-volatile storage element in responseto establishing the fourth voltage on the selected word line todetermine whether the threshold voltage of the second non-volatilestorage element is above or below a second reference voltage.
 12. Thenon-volatile storage device of claim of claim 11 wherein the firstreference voltage is different from the second reference voltage. 13.The non-volatile storage device of claim of claim 11 wherein the firstreference voltage is the same as the second reference voltage.
 14. Thenon-volatile storage device of claim 11, wherein the one or moremanaging circuits establish the second voltage based on a first state ofa plurality of states to which the first non-volatile storage element isbeing programmed by the one or more managing circuits, the one or moremanaging circuits establish the third voltage based on a second state ofthe plurality of states to which the second non-volatile storage elementis being programmed by the one or more managing circuits.
 15. Thenon-volatile storage device of claim 14, wherein the one or moremanaging circuits establish the second voltage on a third bit line ofthe bit lines that is associated with a third NAND string of the NANDstrings, the third NAND string includes a third non-volatile storageelement, the one or more managing circuits establish the third voltageon a fourth bit line of the bit lines that is associated with a fourthNAND string of the NAND strings, the fourth NAND string includes afourth non-volatile storage element, the one or more managing circuitsraise the voltage on the selected word line from the fourth voltage to afifth voltage after sensing the first and second conditions, the one ormore managing circuits bias the first bit line and the third bit line atthe first voltage after sensing the first and second conditions, the oneor more managing circuits sense a third condition of the thirdnon-volatile storage element in response to raising the voltage on theword line to the fifth voltage to determine whether the thirdnon-volatile storage element is programmed to a third data state of theplurality of states, the one or more managing circuits sense a fourthcondition of the fourth non-volatile storage element in response toraising the voltage on the word line to the fifth voltage to determinewhether the fourth non-volatile storage element is programmed to afourth data state of the plurality of states.
 16. A non-volatile storagedevice comprising: a plurality of NAND strings of non-volatile storageelements, the NAND strings include a first NAND string having a firstnon-volatile storage element and a second NAND string having a secondnon-volatile storage element; a plurality of word lines associated withthe plurality of NAND strings; and one or more managing circuits incommunication with the non-volatile storage elements, the one or moremanaging circuits determine whether the threshold voltage of the firstnon-volatile storage element is above or below a first reference voltagewithin a voltage range, the one or more managing circuits determinewhether the threshold voltage of the second non-volatile storage elementis above or below the first reference voltage, the one or more managingcircuits determine a first voltage difference to apply across the firstNAND string based on the threshold voltage of the first non-volatilestorage element, the one or more managing circuits determine a secondvoltage difference to apply across the second NAND string based on thethreshold voltage of the second non-volatile storage element, the firstvoltage difference is different than the second voltage difference, theone or more managing circuits cause the first voltage difference acrossthe first NAND string while causing the second voltage difference acrossthe second NAND string, the one or more managing circuits apply a readvoltage to a selected word line that is associated with the firstnon-volatile storage element and with the second non-volatile storageelement while causing the first and second voltage differences, the oneor more managing circuits sense a first condition of the firstnon-volatile storage element in response to the first voltage differenceand read voltage to determine whether the threshold voltage of the firstnon-volatile storage element is above or below a second referencevoltage within the voltage range, the one or more managing circuitssense a second condition of the second non-volatile storage element inresponse to the second voltage difference and read voltage to determinewhether the threshold voltage of the second non-volatile storage elementis above or below a third reference voltage within the voltage range,the third reference voltage is different from the second referencevoltage.
 17. The non-volatile storage device of claim 16 wherein thevoltage range has an upper voltage and a lower voltage, the firstreference voltage is midway between the lower voltage and the uppervoltage, the second reference voltage is half-way between the firstreference voltage and the upper voltage, the third reference voltage ishalf-way between the first reference voltage and the lower voltage. 18.The non-volatile storage device of claim 16 wherein the one or moremanaging circuits determine a third voltage difference to apply acrossthe first NAND string based on whether the threshold voltage of thefirst non-volatile storage element is above or below the secondreference voltage, the one or more managing circuits determine a fourthvoltage difference to apply across the second NAND string based onwhether the threshold voltage of the second non-volatile storage elementis above or below the third reference voltage, the one or more managingcircuits cause the third voltage difference across the first NAND stringwhile causing the fourth voltage difference across the second NANDstring, the one or more managing circuits apply the read voltage to theword line while causing the third and fourth voltage differences, theone or more managing circuits sense a third condition of the firstnon-volatile storage element in response to the third voltage differenceand read voltage to determine whether the threshold voltage of the firstnon-volatile storage element is above or below a fourth referencevoltage within the voltage range, the one or more managing circuitssense a fourth condition of the second non-volatile storage element inresponse to the fourth voltage difference and read voltage to determinewhether the threshold voltage of the second non-volatile storage elementis above or below a fifth reference voltage within the voltage range,the fifth reference voltage is different from the fourth referencevoltage.
 19. The non-volatile storage device of claim 18 wherein thevoltage range has an upper voltage and a lower voltage, the firstreference voltage is midway between the lower voltage and the uppervoltage, the second reference voltage is half-way between the firstreference voltage and the upper voltage, the third reference voltage ishalf-way between the first reference voltage and the lower voltage, thefourth reference voltage is either midway between the second voltage andthe upper voltage or midway between the first voltage and the secondvoltage, the fifth reference voltage is either midway between the thirdvoltage and the lower voltage or midway between the first voltage andthe third voltage.
 20. A non-volatile storage device comprising: aplurality of NAND strings of non-volatile storage elements, the NANDstrings include a first NAND string having a first non-volatile storageelement and a second NAND string having a second non-volatile storageelement, the non-volatile storage elements each having a gate and asource; a plurality of word lines associated with the plurality of NANDstrings; and one or more managing circuits in communication with thenon-volatile storage elements, the one or more managing circuits programthe non-volatile storage elements to “n+1” data states, each data statecorresponding to a range of threshold voltages, adjacent data stateshaving overlapping ranges of threshold voltages, the one or moremanaging circuits perform “n” sense operations at “n” referencesvoltages that fall within the overlapping ranges of threshold voltagesbetween each of the data states to determine whether the thresholdvoltage of each non-volatile storage element in a subset of thenon-volatile storage elements is greater or less than each of the “n”references voltages, the one or more managing circuits perform anadditional sense operation for each of the “n” sense operations, each ofthe additional sense operations includes simultaneously sensing a firstgroup of non-volatile storage elements in the subset having a thresholdvoltage greater than the “nth” reference voltage at a first offsetvoltage from the “nth” reference voltage while sensing a second group ofnon-volatile storage elements in the subset having a threshold voltageless than the “nth” reference voltage at a second offset voltage fromthe “nth” reference voltage, the first and second offsets are different,the one or more managing circuits sense at the first offset voltage fromthe “nth” reference voltage includes applying a first gate-to-sourcevoltage difference to non-volatile storage elements in the first groupand sensing first conditions of non-volatile storage elements in thefirst group in response to the first gate-to-source voltage differenceto determine whether the threshold voltages of non-volatile storageelements in the first group is above or below the first offset from the“nth” reference voltage, the one or more managing circuits sense at thesecond offset voltage from the “nth” reference voltage includes applyinga second gate-to-source voltage difference to non-volatile storageelements in the second group and sensing second conditions ofnon-volatile storage elements in the second group in response to thesecond gate-to-source voltage difference to determine whether thethreshold voltages of non-volatile storage elements in the second groupis above or below the second offset from the “nth” reference voltage.